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A Comprehensive Overview of the Neural Mechanisms of Light Therapy

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• Published: 09 August 2023
• Volume 40 , pages 350–362, ( 2024 )

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• Xiaodan Huang   ORCID: orcid.org/0000-0001-6222-7508 1 ,
• Qian Tao   ORCID: orcid.org/0000-0002-9725-1871 2 &
• Chaoran Ren   ORCID: orcid.org/0000-0002-6147-8644 1  

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Light is a powerful environmental factor influencing diverse brain functions. Clinical evidence supports the beneficial effect of light therapy on several diseases, including depression, cognitive dysfunction, chronic pain, and sleep disorders. However, the precise mechanisms underlying the effects of light therapy are still not well understood. In this review, we critically evaluate current clinical evidence showing the beneficial effects of light therapy on diseases. In addition, we introduce the research progress regarding the neural circuit mechanisms underlying the modulatory effects of light on brain functions, including mood, memory, pain perception, sleep, circadian rhythm, brain development, and metabolism.

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Introduction

Changes in lighting conditions have broad effects on diverse physiological and behavioral functions, including circadian rhythm, mood, and cognition [ 1 , 2 ]. In humans, light therapy can alleviate depression, promote cognitive function, and relieve pain symptoms [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. It provides a solution for the treatment of brain diseases with the advantages of non-invasiveness, few side-effects, and low cost. Given the acknowledgment of its beneficial effects on brain diseases, the neural mechanisms underlying the beneficial effects of light therapy on brain functions are not well understood. This has caused obstacles to the application and optimization of light therapy in the clinic. Here, we review the progress of human clinical research into light therapy and introduce the basic research progress regarding the circuit mechanisms underlying the modulatory effects of light on brain functions. Through analyzing these research findings, we hope to provide readers with a comprehensive understanding of the modulatory effects of light on brain functions and inspire new ideas for the application and optimization of light therapy.

Clinical Evidence for Light Therapy

Light therapy for depression.

As early as the 1980s, light therapy was first used to treat seasonal affective disorder (SAD) in clinical settings. The results of a meta-analysis that included 19 randomized controlled trials confirmed the effectiveness of light therapy in the treatment of SAD ( d = −0.37, 95% CI: −0.63, −0.12) [ 13 ]. As a cost-effective physical intervention, light therapy has many advantages, such as high safety, mild side-effects, and convenience. To date, light therapy has become the first-line treatment for SAD. In addition to SAD, an increasing number of clinical studies have indicated that light therapy has certain therapeutic effects on various types of non-seasonal depression. For instance, Lam and colleagues conducted a randomized controlled trial on 122 patients with major depressive disorder, and the results showed that the light therapy group ( d = 0.80, 95% CI: 0.28, 1.31) and the combined treatment group (light therapy/fluoxetine) ( d = 1.11, 95% CI: 0.54, 1.64) had significantly superior improvements in outcomes compared to those in the placebo group [ 14 ]. Pierre and colleagues conducted a systematic review of major depressive disorder and concluded that light therapy and antidepressants were equally effective (SMD = 0.19, 95% CI: −0.08, 0.45) and that light therapy combined with antidepressants was significantly better than monotherapy with antidepressants (SMD = 0.56, 95% CI: 0.24, 0.88) [ 15 ]. An open trial suggested that 3 weeks of bright light therapy is efficacious for antepartum depression, and the benefits were seen at the 2-week follow-up assessment [ 16 ]. Our previous study also suggested that 8 weeks of bright light therapy had beneficial effects in treating subthreshold depression in a college student sample [ 17 ]. In summary, the beneficial effects of light therapy have been confirmed in several types of depression, but the optimal parameters for this intervention are controversial. However, guidelines for the clinical applications of light therapy in depression are lacking.

Light Therapy for Cognition

Light is essential for many cognitive tasks. A Cochrane meta-study that included three randomized controlled trials and two before-after trials with 282 daytime worker participants, showed that bright illumination in the environment improves alertness [ 18 ]. Our recent meta-study showed that light exposure in laboratory settings improves both objective and subjective alertness, and the correlated color temperature of light treatment is a key parameter to determine the beneficial effects [ 19 ]. The potential application of light therapy for cognitive disorders has been increasingly recognized in the last decade [ 20 ]. A recent randomized placebo-controlled trial of blue wavelength exposure was conducted in 32 adults with mild traumatic brain injury [ 21 ]. Compared with the placebo, blue light led to reduced daytime sleepiness and improved executive functioning, which was associated with brain changes in the retinohypothalamic system [ 21 ]. A Cochrane meta-study found no adequate evidence of the effectiveness of light therapy in managing cognitive function in dementia [ 22 ]. A recent study using a crossover design suggested that a whole-day lighting scheme that follows the natural light/dark cycle improves cognitive performance in older adults, as indexed by the trail-making test (assessing executive function) and the digit symbol substitution test (assessing processing speed and attention) [ 23 ]. Another recent clinical trial demonstrated the efficacy of bright light therapy in improving objective cognitive function in the survivors of hematopoietic stem cell transplants [ 24 ]. Blue-enriched light therapy has been found to be beneficial in improving general cognitive function in patients with mild and moderate Alzheimer’s disease (AD) [ 25 ]. Overall, it seems that light intervention is useful for improving alertness in healthy individuals, but the evidence for its effect on other cognitive functions, such as memory, attention, and executive function, is not adequate. Investigations on the effectiveness of light therapy in patients with cognitive disorders are scarce, and conflicting results have been reported. We speculate that at least three reasons contribute to these inconsistent results. First, the stage and severity of cognitive impairment should be considered. Second, outcomes should be expanded to many cognitive domains, rather than general cognition. Finally, light therapy may be a good supplementary intervention to traditional cognitive interventions. In the future, more studies are required to investigate the efficacy of light therapy on different aspects of cognition.

Light Therapy for Pain

In clinical practice, the common approaches to treating chronic pain include opioid analgesic medication [ 26 ], cognitive behavioral therapy [ 27 , 28 ], meditation, and acupuncture [ 29 , 30 ]. However, these approaches have various limitations. For instance, pain treatments are not always effective, have side-effects, and are expensive [ 31 ]. Although clinical evidence documenting the relationship between light therapy and pain is scarce, light therapy is a promising, available, and safe intervention to manage chronic pain. A prospective study evaluated the effect of natural sunlight on pain among patients undergoing spinal surgery [ 9 ]. The results showed that patients staying on the bright side of the room had decreased pain, analgesic medication use, and pain medication cost than those staying on the dim side [ 9 ]. Patients with fibromyalgia and US military veterans often report experiencing pain; Burgess and colleagues conducted three studies on the effect of bright light therapy in these two populations. Their first study indicated that bright light therapy improves pain sensitivity (less sensitive to pain) in female patients with fibromyalgia [ 10 ]. The second study suggested that morning bright light therapy reduces pain intensity, pain behavior, and the thermal pain threshold in US veterans with chronic low back pain [ 11 ]. Importantly, phase advances in circadian timing were significantly associated with increased pain tolerance in both studies. A further study evaluated volatility and showed that bright light therapy was related to participants experiencing fewer “pain flares” [ 32 ]. Overall, several limitations, such as small sample sizes and the lack of a double-blind design, were noted for the above studies. More future clinical studies of high quality are needed to document the evidence for light therapy and pain.

Light Therapy for Sleep and Circadian Rhythms

Light, sleep, and circadian rhythms have closely interacted to help organisms to adapt to the environment [ 2 ]. First, light influences the suprachiasmatic nucleus (SCN) that controls circadian rhythms. Second, light suppresses melatonin secretion, which is crucial for the regulation of circadian rhythms and sleep. Consequently, light therapy has been applied as a potential treatment for sleep disorders. For instance, a randomized control study suggested that light and dark exposure in shift work nurses improves their insomnia symptoms [ 33 ]. Morning bright light exposure advances the circadian rhythms in patients with insomnia [ 34 ] and delays sleep phase syndrome [ 35 ]. However, one study reported limited effects of light therapy on individuals >55 years old with mild early-morning awakenings [ 36 ]. A meta-study that included 53 studies and 1154 participants found beneficial effects of light therapy on sleep problems in general, including insomnia, and the sleep problems associated with AD and dementia, but limited effects on circadian rhythm sleep disorders [ 37 ]. Furthermore, combined bright light and melatonin therapies have been shown to be superior to single therapy in advancing the phase in healthy individuals and improving sleep outcomes in elderly populations with cognitive decline [ 38 ]. Light therapy also improves sleep quality in patients with Parkinson’s disease [ 39 ]. It has been noted that the parameters of light therapy greatly influence the sleep outcomes and circadian rhythms. A consistent finding is that light exposure during daytime has beneficial effects on sleep, whereas light exposure at night has adverse effects. More investigations are needed to explore the effects of light characteristics on sleep and circadian rhythms.

Human Neuroimaging Studies Related to Light Therapy

Although light therapy has been widely reported to be beneficial in clinical studies (Fig. 1 ), the underlying mechanisms remain largely unknown. The clinical practice of light therapy in the treatment of depression is mainly based on the phase-shifting hypothesis, which indicates that light has a role in re-synchronizing circadian rhythms [ 40 ]. However, other mechanisms may also mediate the antidepressant effect of light therapy based on three lines of evidence. First, light therapy requires a higher intensity (>5000 lx) to treat depression than to alter circadian rhythms (~120 lx). Second, the relief of depressive symptoms is not consistently associated with changes in circadian rhythm [ 40 , 41 ]. Finally, light therapy has been shown to be efficacious in treating not only seasonal depression but also many types of non-seasonal depression [ 14 , 42 , 43 , 44 , 45 ]. Pertinently, not all types of non-seasonal depression are related to phase shifting.

Clinical studies of light therapy. The clinical studies of light therapy in the regulation of mood, cognition, and pain. SAD, seasonal affective disorder; BD, bipolar disorder; SD, subthreshold depression; PD, Parkinson's disease; AD, Alzheimer’s disease.

Neuroimaging techniques have provided some clues for the neural mechanisms underlying the beneficial effects of light therapy. An early study collected functional magnetic resonance imaging (fMRI) data while participants attended to auditory and visual stimuli in darkness following short-term light exposures in the scanner [ 46 ]. The results revealed increased activation in the occipito-parietal network and decreased activation in the hypothalamus [ 46 ]. By using an auditory oddball task, another fMRI study found that short-term exposure to bright white light induced dynamic responses in the posterior thalamus and subcortical regions supporting attentional effects [ 47 ]. A further fMRI study evaluated the wavelength effect of light exposure on an auditory working memory task [ 48 ]. Compared with green light exposure, blue light exposure led to changes in the left intraparietal sulcus, supramarginal gyrus, right insula, left middle frontal gyrus, and left thalamus [ 48 ]. All of the above studies demonstrated non-visual responses to short-term light exposure in the human brain within scanners. However, neuroimaging studies on light therapy are few. We only identified two relevant studies. Fisher and colleagues conducted an fMRI study on healthy individuals with an emotional faces paradigm [ 49 ]. The results demonstrated that a three-week bright-light intervention changed amygdala-prefrontal reactivity and functional coupling, which was partly moderated by the 5-HTTLPR (serotonin-transporter-linked promoter region) genotype [ 49 ]. This important finding has provided the first evidence indicating that serotonin and the threat-related circuit may underlie the effects of light therapy in humans. Another study used resting-state fMRI to investigate the neural mechanisms underlying two-week morning bright light exposure in individuals with sleep disturbances [ 50 ]. The results demonstrated decreased functional connectivity in the anterior insular and frontal opercular regions that were correlated with decreased sleep latency in a bright light group [ 50 ]. More longitudinal neuroimaging studies are needed to reveal the neural mechanisms underlying the non-visual effects of light.

Animal Studies

As noted above, clinical studies of light therapy have mainly focused on the utility of this treatment for mood-, cognition-, pain-, and sleep-related disorders. Due to the complexity of the biological mechanisms underlying these disorders and the limitations of technologies that can be used in humans, the neural mechanisms of light therapy are poorly understood. To further analyze the neural mechanism of light therapy, experimental animals are indispensable research subjects. Unveiling the neuronal basis for the effects of light on brain functions in animal models constitutes a promising step toward new treatments for neuropsychiatric disorders.

Photosensitive Cells in the Retina

In mammals, the effects of external light on brain functions are mainly mediated by specific visual circuits [ 51 ]. At the beginning of the visual circuits, photosensitive cells in the retina convert light information into bioelectrical information that is transmitted to different brain regions [ 51 , 52 , 53 ]. In the mammalian retina, there are three types of light-sensitive cells: rods, cones, and intrinsic photosensitive retinal ganglion cells (ipRGCs) [ 54 , 55 ]. Among these, rods and cones are conventional photoreceptors that transmit light signals to RGCs through bipolar cells. Owing to the expression of the photosensitive protein melanopsin, ipRGCs not only receive light signals transmitted from conventional photoreceptors but also respond directly to external light stimuli even in the absence of rods and cones [ 55 , 56 , 57 , 58 , 59 , 60 ]. Studies have found that ipRGCs are highly conserved in several species, including humans [ 52 , 61 , 62 ], and they transmit light information to multiple brain regions to regulate brain functions unrelated to image-forming vision, including the circadian rhythm [ 1 ], cognition [ 20 ], and mood [ 2 ]. The discovery of ipRGCs also opened up a new field of research, namely, the non-image-forming visual functions of light.

IpRGCs can be divided into different subtypes according to their morphological characteristics, such as the size of the somata, the ramification pattern of dendrites stratified in the inner plexiform layer of the retina, and the size and complexity of the dendritic field. At present, it is known that mouse ipRGCs are divided into six subtypes, M1-M6 [ 52 ]; rat ipRGCs are divided into five subtypes, M1-M5 [ 61 ], and human ipRGCs are known to have four subtypes, M1-M4 [ 62 ]. In addition to morphological differences, ipRGCs vary in other aspects. For example, M1-ipRGCs have the highest level of melanopsin expression, while M4-ipRGCs express melanopsin at a very low level [ 63 , 64 ]. In addition, different subtypes of ipRGC differ in their response to light and their projection patterns to brain regions [ 61 , 65 , 66 ] (Fig. 2 ). It is precisely because of the cellular heterogeneity of ipRGCs that they mediate the effects of light on diverse brain functions, such as mood, memory, pain perception, sleep, circadian rhythm, brain development, and metabolism.

The efferent projections of ipRGCs. The main brain targets of ipRGCs. IPL, inner plexiform layer; ipRGCs, intrinsic photosensitive retinal ganglion cells; AH, anterior hypothalamus; BST, bed nucleus of the stria terminalis; SPZ, subparaventricular zone; VLPO, ventrolateral preoptic area; SON, supraoptic nucleus; SCN, suprachiasmatic nucleus; MA, medial amygdaloid nucleus; pHb, perihabenular nucleus; LHb, lateral habenula; SC, superior colliculus; PAG, periaqueductal gray; OPN, olivary pretectal nucleus; dLGN, dorsal lateral geniculate nucleus; IGL, intergeniculate leaflet; vLGN, ventral lateral geniculate nucleus.

Circuit Mechanisms Underlying the Effects of Light on Mood

Clinical studies of SAD patients have found that rapid tryptophan depletion reverses the antidepressant effects of light therapy [ 67 , 68 , 69 ], suggesting that light information might influence depressive-like behaviors through specific circuits linking the retina and the midbrain monoaminergic centers. The lateral habenula (LHb), part of the epithalamus, is a highly conserved nucleus across species, and it regulates the flow of information from the limbic system to the midbrain monoaminergic centers [ 70 ]. Studies of rodents have suggested that light influences neural activity in the LHb [ 2 , 71 ]. Therefore, visual circuits related to the LHb might mediate the effects of light on depressive-like behaviors. Consistent with this view, our recent study of mice found that the ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL) are important relay regions linking the retina and LHb [ 72 ]. Bright light signals transmitted by a subset of M4-ipRGCs activate a subset of GABAergic neurons in the vLGN/IGL, which in turn inhibit the activity of excitatory neurons in the LHb. We found that specific activation of RGCs projecting to the vLGN/IGL, activation of LHb-projecting vLGN/IGL neurons, or inhibition of postsynaptic LHb neurons is sufficient to decrease the depressive-like behaviors evoked by long-term exposure to aversive stimuli or chronic social defeat stress. Furthermore, we demonstrated that the activation of the retina-vLGN/IGL-LHb pathway is needed for the anti-depressive effects of bright light treatment [ 72 ]. These results provide a potential mechanistic explanation for the antidepressant effects of bright light treatment.

Unlike the role of the LHb in mediating the beneficial effects of light on depressive-like behaviors, recent studies have shown that the perihabenular nucleus (pHb) located in the dorsal thalamus mediates the negative effects of light on depressive-like behaviors [ 73 , 74 ]. For example, Fernandez and colleagues found that long-term exposure to an ultradian light : dark cycle (T7 cycle : 3.5 h light and 3.5 h dark) accompanied by pHb activation increases depressive-like behaviors in mice [ 73 ]. They further demonstrated that the depression-inducing effects of a fast ultradian light cycle are mediated by an M1-ipRGC→pHb→mPFC pathway. In addition to the ultradian light/dark cycle, An and colleagues found that excessive light exposure at night (LAN) also increases depressive-like behaviors without disrupting the circadian rhythm [ 74 ]. Importantly, An and colleagues demonstrated that the depression-inducing effects of LAN are mediated by a visual circuit consisting of M1-ipRGCs, the pHb, and the nucleus accumbens. Those two studies provide direct evidence that the pHb-related visual circuits play a pivotal role in mediating the negative effects of light on depressive-like behaviors.

However, the relevant question is about the idea that plenty of light during the daytime should also activate the M1-ipRGC→pHb pathway, which can increase depressive-like behaviors. Why does daytime light exposure (such as bright light therapy) have an antidepressant effect? One possible explanation is that the excitability of pHb neurons is higher at night than in the daytime and tends to conduct nighttime light information [ 74 ]. Taken together, recent studies conducted in rodents have suggested that the pHb acts as a valve controlled by the circadian rhythm, specifically mediating the regulation of negative mood by nocturnal light messages, and daytime light exposure alleviates depression through the LHb-related visual circuits (Fig. 3 A).

Circuit mechanisms underlying the effects of light treatment. A Bright light treatment (LT) in the daytime induces antidepressant effects through the M4 ipRGC→vLGN/IGL→LHb pathway (orange), while light at night (LAN) increases depressive-like behaviors through the M1 ipRGC→pHb→NAc pathway (blue). B LT promotes spatial memory through the M4 ipRGC→vLGN/IGL→Re pathway (orange), while a fast ultradian light cycle (T7, 3.5 h light, and 3.5 h dark) impairs memory (blue) through the M1 ipRGC→SCN pathway. (C) LT exerts antinociceptive effects through the M4 ipRGC→vLGN/IGL→v/lPAG pathway, while green light treatment (GLT) has antinociceptive effects through the cRGC→vLGN→DRN pathway and V2M→ACC pathway. RGC, retinal ganglion cell; vLGN/IGL, ventral lateral geniculate nucleus and intergeniculate leaflet; PHb, perihabenular nucleus; LHb, lateral habenula; NAc, nucleus accumbens; SCN, suprachiasmatic nucleus; Re, reunions nucleus; v/lPAG, lateral and ventral lateral periaqueductal gray; DRN, dorsal raphe; V2M, the secondary visual cortex; ACC, anterior cingulate cortex.

Circuit Mechanisms Underlying the Effect of Light on Memory

Memory dysfunction is a typical characteristic of AD, the pathogenesis of which is complex, including amyloid-β (Aβ) deposition, tau accumulation, and disrupted network oscillation [ 75 , 76 ]. Studies conducted in mice have shown that light flickering at 40 Hz (gamma entrainment using sensory, GENUS) drives gamma oscillations in the visual cortex [ 77 ]. The authors demonstrated that the long-term application of GENUS improves memory in several mouse models of AD, including the CK-p25, P301S, and 5xFAD models [ 78 ]. In addition, by training neurons to oscillate at gamma frequency, GENUS triggers the engulfment activity of microglia, while decreasing Aβ deposition and tau accumulation in several brain regions, including the visual cortex, hippocampus (HPC), and prefrontal cortex (PFC) [ 77 , 78 ]. Moreover, GENUS enhances synaptic function in neurons [ 78 ]. These findings uncover a previously unappreciated function of gamma rhythms in recruiting both neural and glial responses to attenuate AD-associated pathology. However, the circuit mechanisms underlying the effects of GENUS on memory are still unknown. It should also be noted that a recent study found that 40 Hz flickering light does not entrain gamma oscillations and suppress Aβ in the brains of APP/PS1 and 5xFAD mouse models of AD [ 79 ]. Thus, more detailed work is needed to investigate the effects of GENUS and the underlying mechanism.

Accumulating evidence suggests that bright light has beneficial effects on memory. In humans, brighter illumination during the day improves cognitive performance [ 4 , 5 , 6 , 7 , 8 ], and bright light therapy appears to attenuate cognitive deterioration in early-stage dementia [ 22 , 80 ]. In rodents, bright light has been shown to enhance fear and spatial memory [ 81 , 82 , 83 ]. However, the neural mechanisms underlying the effects of bright light on memory are not well understood. A recent study by our group probed the circuit mechanisms underlying the beneficial effects of bright light treatment on spatial memory [ 84 ]. We found that long-term exposure to bright light promotes the spatial memory tested in the novel location test and Morris water maze test in young WT mice (3 months old); this is accompanied by the increased power of gamma oscillation in the HPC during the performance of a spatial memory-related task [ 84 ]. These results suggest that bright light treatment might promote spatial memory through certain visual circuits associated with the HPC. However, it is well established that the HPC does not receive direct retinal projections. To unveil the neural circuits underlying the beneficial effects of bright light on spatial memory, we conducted whole-brain c-Fos mapping to determine the neural substrates that can be regulated by bright light. We found that bright light increases c-Fos expression in the nucleus reunions (Re) [ 84 ], which interacts with the HPC and plays an important role in the regulation of memory [ 85 , 86 , 87 ]. These results suggest that the spatial-memory-promoting effects of bright light might be mediated by a visual circuit linking the retina and Re. Consistent with this view, we found that bright light promotes spatial memory through activating an M4-ipRGC→vLGN/IGL→Re pathway [ 84 ] (Fig. 3 B). These results reveal a dedicated subcortical visual circuit that mediates the spatial-memory-promoting effects of bright light treatment.

It should be noted that both GENUS and bright light promote spatial memory and increase gamma oscillation in the HPC. One might expect that bright light should also improve spatial memory in a mouse model of AD. However, we found that long-term exposure to bright light does not significantly promote spatial memory in 6-month-old 5xFAD mice [ 84 ]. In addition, unlike GENUS, bright light alone does not directly increase gamma oscillation in the HPC [ 84 ]. The HPC gamma oscillation detected during the spatial memory-related task may be too weak to reduce the amyloid load and reverse the deficits in spatial memory of 5xFAD mice. On the other hand, GENUS does not show beneficial effects on spatial memory in WT mice [ 78 ], suggesting that the neural mechanisms underlying the spatial-memory-promoting effects of bright light treatment and GENUS are different.

In addition to the beneficial effects of light on memory, certain light patterns can impair memory. Fernandez and colleagues found that a fast ultradian light cycle (T7) not only increases depressive-like behaviors but also results in memory deficits, accompanied by decreased cellular plasticity in the HPC [ 73 ]. They demonstrated that the negative effects of the fast ultradian light cycle on memory are mediated by an M1-ipRGC→SCN pathway (Fig. 3 B). Given that the SCN plays a pivotal role in the regulation of circadian rhythm, these results suggest that a disrupted circadian rhythm might underlie fast ultradian light cycle-induced memory deficits. In addition, studies conducted in grass rats, a diurnal rodent species, found that dim light exposure (5 lx) at night impairs spatial memory accompanied by a decreased dendritic length in the dentate gyrus and basilar CA1 dendrites [ 88 ], without affecting the circadian rhythm tested in the wheel-running test. This result suggests that certain visual circuits unrelated to the regulation of circadian rhythm might also mediate the negative effects of LAN on memory.

Circuit Mechanisms Underlying the Effect of Light on Pain Perception

Clinical studies have shown that light therapy can relieve pain symptoms in patients with chronic low back pain, headache, fibromyalgia, and postoperative pain [ 9 , 10 , 11 , 12 ]. Consistent with this, animal studies have shown that light also has antinociceptive effects in rodents [ 89 , 90 ]. However, the precise circuits that mediate the effects of light on nocifensive behaviors remain unclear. Our recent study in mice found that GABAergic neurons in the vLGN/IGL directly synapse with GABAergic neurons in the lateral and ventral lateral periaqueductal gray (l/vlPAG) [ 91 ], which is an important part of the ascending pain conduction and descending pain regulation systems [ 92 , 93 , 94 , 95 ]. We found that specific activation of the vLGN/IGL not only induces inhibitory postsynaptic currents in l/vlPAG GABAergic neurons but also reduces the excitatory effects of pain-related stimuli on l/vlPAG GABAergic neurons [ 91 ], suggesting that the vLGN/IGL→l/vlPAG pathway might modulate pain-related behaviors. Consistent with this, we found that specific activation of l/vlPAG-projecting vLGN/IGL neurons or inhibition of l/vlPAG postsynaptic neurons not only elevates the pain threshold in WT mice but also improves the pain-related symptoms in mouse models of pain [ 91 ], indicating the antinociceptive effects induced by activation of the vLGN/IGL→l/vlPAG pathway. We further found that the vLGN/IGL→l/vlPAG pathway receives direct innervation from RGCs [ 91 ], suggesting that light information transmitted by the retina→vLGN/IGL→l/vlPAG pathway might modulate pain-related behaviors. In support of this view, we demonstrated that the antinociceptive effects of bright light treatment are dependent on the activation of the retina→vLGN/IGL→l/vlPAG pathway [ 91 ] (Fig. 3 C).

In addition to bright light, exposure to green light also has antinociceptive effects in both humans and rodents [ 89 , 96 , 97 , 98 ]. A recent study in mice found that activation of the retina→vLGN→dorsal raphe (DRN) pathway is needed for the antinociceptive effects of green light [ 99 ]. It is worth noting that conventional photoreceptors, but not ipRGCs, are required for the antinociceptive effects of green light, suggesting that conventional RGCs can also mediate the antinociceptive effects of light (Fig. 3 C). Interestingly, another study revealed that a circuit linking the visual cortex and the anterior cingulate cortex circuit plays a pivotal role in mediating the antinociceptive effects of green light (Fig. 3 C) [ 100 ]. Given that the vLGN-related visual circuits are also needed for green light analgesia, it would be interesting to investigate the mechanisms underlying the interaction between vLGN-related visual circuits and the cerebral cortex.

Circuit Mechanisms Underlying the Effects of Light on Sleep and Circadian Rhythms

Sleep is regulated by both homeostatic mechanisms and circadian rhythm [ 101 ]. Light can modulate sleep by entraining the circadian rhythm [ 2 ]. The SCN located in the hypothalamus is the central pacemaker of the circadian timing system [ 102 ]. Since ipRGCs project directly to the SCN, it was assumed that ipRGCs play a major role in mediating the effects of light on circadian rhythms. Consistent with this view, it has been found that specific ablation of ipRGCs results in the complete loss of circadian photoentrainment [ 103 , 104 , 105 , 106 ]. However, abolishing the intrinsic sensitivity to light of ipRGCs by removing the gene encoding the photosensitive protein melanopsin does not significantly affect circadian photoentrainment [ 107 , 108 ]. Given that ipRGCs also receive light signals transmitted by rods and cones, it is plausible that rods/cones phototransduction via ipRGCs is sufficient for circadian photoentrainment. In support of this hypothesis, studies have shown that light does not regulate the circadian rhythm tested in the wheel-running test in mice lacking both rods/cones and melanopsin [ 109 ].

In addition to the SCN, other brain regions can also mediate the effects of light on sleep. It has been reported that sleep-related brain regions, such as the ventrolateral preoptic area (VLPO) and the superior colliculus (SC), are directly innervated by ipRGCs [ 66 ], and the changed neural activity in the VLPO and SC might mediate the acute light-induced sleep at night in mice [ 110 , 111 ]. In addition, Zhang and colleagues found that acute dark exposure during the daytime induces wakefulness in mice by activating a retina→SC→VTA pathway [ 112 ]. They demonstrated that a dark pulse administered during the daytime disinhibits VTA dopaminergic neurons by inhibiting SC GABAergic neurons, which consequently leads to increased wakefulness and reduced sleep [ 112 ]. Thus, both circadian-related and circadian-unrelated mechanisms underlie the modulatory effects of light on sleep.

Circuit Mechanisms Underlying the Effects of Light on Brain Development

The light sensation is crucial for brain development [ 113 ]. For example, raising animals in complete darkness or depriving one or both eyes at early developmental stages leads to significant shrinkage of thalamic axonal arbors in the visual cortex [ 114 ]. In 2014, Yu and colleagues found that dark rearing from birth reduces the excitatory synaptic transmission in multiple sensory cortices, and this impairment can be rescued by elevating the release of oxytocin from the paraventricular nucleus (PVN) through an enriched environment during the early days of dark rearing [ 115 ]. These findings suggest that the neuropeptide oxytocin is a key molecule in mediating the effects of light on brain development. However, the visual circuits that mediate the effects of light on oxytocin secretion still need to be determined. It is well established that ipRGCs become light sensitive much earlier than rods and cones, and play a pivotal role in mediating the earliest light sensation in mammals [ 116 ]. This suggests that visual circuits associated with ipRGCs might be important for mediating the effects of light on brain development. Consistent with this, a recent study by Hu and colleagues found the ipRGC→supraoptic nucleus (SON)→PVN pathway mediates light-promoted brain development [ 117 ]. They demonstrated that light-induced activation of ipRGCs increases the release of oxytocin from the SON and PVN into the cerebrospinal fluid [ 117 ]. Importantly, they found that a lack of ipRGC-mediated, light-promoted early cortical synaptogenesis compromises learning ability in adult mice [ 117 ]. These results provide new insight into the circuit mechanisms of the impacts of light on brain development.

In addition to the development of learning, early light sensation also influences non-photic circadian entrainment. Fernandez and colleagues established a time-restricted feeding model in mice by limiting food access to a 7-h period (ZT4-ZT11) [ 118 ]. They found that the innervation of ipRGCs at early postnatal stages influences IGL neurons that express neuropeptide Y, guiding the assembly of a functional IGL→SCN pathway [ 118 ]. Furthermore, they demonstrated that ablation of ipRGCs during early postnatal stages alters the connectivity between the IGL and SCN, reduces the expression of neuropeptide Y in the SCN, and results in decreased food-anticipatory activity in adult mice [ 118 ]. This result provides direct evidence that light sensation in the early postnatal period affects entrainment to time-restricted feeding through the ipRGC→IGL→SCN pathway.

Circuit Mechanism Underlying the Effects of Light on Metabolism

Public health studies have shown that a disrupted circadian rhythm is highly relevant to metabolic diseases, including obesity [ 119 , 120 ], diabetes [ 121 ], and cardiovascular disease [ 122 ]. Animal studies have also found that a circadian rhythm disrupted by varying light patterns affects metabolism [ 123 , 124 ]. For example, long-term exposure to light at night alters internal hormonal rhythms [ 125 , 126 , 127 ] and thereby influences glucose metabolism [ 128 , 129 ]. These studies suggest that light regulates metabolism by influencing circadian rhythm. In addition to regulating metabolism by affecting circadian rhythm, Meng and colleagues found that acute light exposure decreases glucose tolerance in mice through a retina→hypothalamus→brown adipose tissue (BAT) pathway [ 130 ]. They demonstrated that ipRGCs directly innervate vasopressin neurons in the SON, which project to the PVN and then to the GABAergic neurons in the solitary tract nucleus, and eventually to BAT. Light-induced activation of this pathway blocks the adaptive thermogenesis in BAT, thereby decreasing glucose tolerance. Moreover, they found that acute light exposure (400 lx) during the daytime or nighttime also decreases glucose tolerance in humans, and these effects are probably involved in the inhibition of BAT-mediated adaptive thermogenesis. These results unveil a visual circuit linking the retina and BAT that mediates the effect of light on glucose metabolism. It would be interesting to test whether a light therapy paradigm targeting this visual circuit can alleviate the symptoms of metabolic disorders, such as obesity.

In addition to regulating metabolism through specific visual-related circuits, Richard A. Lang’s group found that light also regulates the adipose tissue-mediated adaptive thermogenesis through two non-visual-related circuits. In one study, they found that blue light directly activates encephalopsin (OPN3, a blue-light-responsive opsin) located in mouse adipocytes, which in turn increases thermogenesis [ 131 ]. In another study, they found that violet light (380 nm) directly activates the deep brain photosensitive protein OPN5 in the hypothalamic preoptic area, and then suppresses BAT thermogenesis [ 132 ]. These opposing activities of OPN3 and OPN5 on thermogenesis raise the hypothesis that non-visual-related circuits decode light information to help the organism to maintain homeostasis through calibrating BAT activity appropriate to the time of day.

Conclusions and Prospects

Growing evidence derived from both clinical trials and animal studies supports the beneficial effects of light therapy on multiple brain functions, including mood, cognition, sleep, and pain. However, the clinical application of light therapy has been faced with difficulties due to the unclear mechanism of action. As noted above, due to advances in research technologies, tremendous progress has been made in deciphering the circuit mechanisms of the effects of light on brain functions, such as mood, memory, pain perception, sleep, circadian rhythm, brain development, and metabolism. Several dedicated neural circuits have been demonstrated to mediate the beneficial and negative effects of light on brain functions. This experimental evidence not only provides a new theoretical basis for the application of light therapy in populations but also provides a new idea for developing novel light therapy strategies targeting specific visual circuits. Despite all these exciting advances, several key questions remain to be addressed.

Multiple key components of the neural circuit for non-visual light signal processing have been identified in the vLGN/IGL. However, an understanding of the vLGN/IGL-related circuits for light signal processing is not complete. For example, we found that vLGN/IGL neurons co-release GABA and glutamate and can be co-labeled by several traditional neural markers [ 91 ]. This suggests that vLGN/IGL neurons cannot be accurately classified by traditional neuron classification criteria. Fine-typing of vLGN/IGL neurons by single-cell transcriptome sequencing, thus revealing the neural connections and functions of different vLGN/IGL neural subtypes, is of great importance for further understanding the circuit mechanisms underlying the modulatory effects of light on brain functions.

Different light patterns can have opposite effects on the same brain function. For example, daytime bright light exposure may have antidepressant effects, while irregular light exposure may increase depression. It is important to establish new light therapy paradigms that can effectively activate specific visual circuits based on the unique light response properties of different visual circuits. This is crucial for the development of novel therapeutic strategies targeting the visual system to improve neurological diseases.

Finally, our knowledge of the effects of non-visual stimuli on the visual system is still limited and requires further investigation. For example, can changes in mood modulate the morphological and physiological properties of certain visual circuits? Can these effects impact the efficiency of light therapy? Systematic investigation of these questions will not only help to further understand the neural mechanisms of light therapy but also provide guidance for the development of new light therapy strategies.

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Acknowledgements

This review was supported by grants from the National Natural Science Foundation of China (32171010 and 32100820), STI2030-Major Projects (2021ZD0203100), and the Guangdong Basic and Applied Basic Research Foundation (2023B1515040010).

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Xiaodan Huang & Chaoran Ren

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Huang, X., Tao, Q. & Ren, C. A Comprehensive Overview of the Neural Mechanisms of Light Therapy. Neurosci. Bull. 40 , 350–362 (2024). https://doi.org/10.1007/s12264-023-01089-8

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Effects of Light on Attention and Reaction Time: A Systematic Review

Rostam golmohammadi.

1 Department of Occupational Health and Safety Engineering, School of Public Health, Hamadan University of Medical Sciences, Hamadan, Iran

Hanieh Yousefi

2 Department of Ergonomics, School of Public Health, Hamadan University of Medical Sciences, Hamadan, Iran

Negar Safarpour Khotbesara

Abbas nasrolahi.

3 Research Center for Prevention of Psychosocial Injuries, Ilam University of medical science, Ilam, Iran

Nematullah Kurd

Background: Accuracy, speed, efficiency, and applicability of activities in the workplace are among the most important effective factors on people's productivity, which is in turn affected by environmental factors, such as light. Therefore, the present research aimed to review the studies performed about the effects of light on attention and reaction time.

Sudy Design: A systematic review.

Methods: This review study systematically searched articles from 2000-2019 in databases of Google Scholar, ISC, SID, Magiran, Web of Science, Science Direct, PubMed, and Scopus using keywords of light, lighting, attention, and reaction time. The titles and abstracts of articles containing relevant results over the past 20 years were extracted. Thereafter, they were categorized and analyzed according to the title, author name, publication year, study method, study type, and evaluation results.

Results: Based on the results, the light with shorter wavelengths, higher intensity, and higher color temperature led to suppressed melatonin, higher consciousness, less somnolence, increased attention, and faster reaction time. Simultaneous exposure to harmful levels of environmental factors affects cognitive and physiological parameters, acting independently with a separate mechanism or synergistically with a similar mechanism. The best light in the regulation of psychological, biological, and cognitive processes is bright daylight in the morning with a short wavelength, high intensity, and more lasting effects.

Conclusion: As evidenced by the obtained results, light is a powerful modulator of non-visual performance in cognitive tasks. The wavelength, color temperature, and light intensity modulate brain responses to cognitive tasks, including attention and reaction time. Therefore, these parameters, along with personal and environmental factors, should be considered in designing and using light.

Introduction

Today, modern technologies have changed the working environment, creating more visual and cognitive needs than just physicalones 1 . Based on the studies conducted in recent decades, good illumination conditions modulate human needs regarding working, economic, environmental, and design-architectural requirements. Human performance, apparent space, safety, health, and well-being are improved by taking advantage of good lighting conditions 2 . Accuracy, speed, efficiency, and applicability of activities in the workplace are among the most effective factors on people's productivity, which is in turn affected by environmental factors, such as light 3 . The human visual system does not work optimally in poor lighting which leads to information loss, increased errors, and decreased performance 4 .

Numerous studies have demonstrated that proper lighting exerts a positive impact on work performance, reducing accident rates. Moreover, inadequate lighting increases eye strain, reduces performance, and leads to accidents 4 - 6 . Human factor research on lighting has largely focused on light visual aspects, as well as visual disturbance and performance. Evidence on the non-visual, psychological, and biological effects of light has recently been presented 7 .They revealed that different lighting conditions significantly affect many non-visual functions, such as physiological and psychological mechanisms, and biological-cognitive processes, such as Circadian Rhythm, consciousness, core body temperature, hormone secretion, and sleep 8 - 10 . Furthermore, several laboratory studies have pointed out that exposure to higher levels of illumination leads to lower melatonin secretion, increased physiological arousal, higher consciousness, as well as improved continuous attention and cognitive function 11 , 12 .

Attention and reaction time are among the important human cognitive indices. Attention is a cognitive process defined as a selective focus on one aspect of the environment while ignoring others 13 . The word "attention" can be defined in accordance with the number of errors made during a test. Accordingly, more careful attention during the test leads to fewer errors and vice versa. Furthermore, there is a close relationship between attention and reaction time 14 . That is to say, the higher levels of attention result in a shorter reaction time, and the opposite is also true. Reaction time is the time elapsed between understanding a situation and the response provided by an individual 15 . In humans, it may last from 0.5-> 3 sec, depending on the type of activity, attention, and consciousness 16 , 17 .

Some studies assessed the effect of lighting on cognitive functions; nonetheless, they have not reached a clear and definite conclusion 18. According to the aforementioned issues, although various studies have been conducted on cognitive functions and their importance, there is a dearth of research pertaining to the effect of inappropriate lighting on cognitive functions, including attention and reaction time. Attention and reaction time have a significant role to play in human errors and the occurrence of accidents; therefore, it is highly important to analyze the influential factors affecting them in the workplace.

This review study investigated the effects of lighting on attention and reaction time. To collect the required data, a query was conducted on six available electronic databases, including Google Scholar, ISC, SID, Magiran, Web of Science, Science Direct, PubMed, and Scopus. The search was performed using the keywords of light, lighting, attention, and reaction times. At each stage, the searched articles in each database were entered into the endnote software. In the first stage, a total of 187 documents related to the topic were entered into the software. In the second stage, according to the framework selected for the study based on a review of published studies from 2000-2019, the relevant documents before this period were deleted, yielding 101 articles. Since many of the records found in various databases were indexed, duplicates were inevitable. Therefore, in the third stage, the duplicates were removed; as a result, 90 documents remained for review. In the next step, the titles of these articles were carefully reviewed, and 21 irrelevant ones were deleted. After the revision of their abstract, another 30 documents were excluded from the assessment due to their irrelevant methodology. Following that, the complete file of the remaining articles was received; however, the full text of three articles could not be accessed, and they were excluded from the review process. The examination of the full texts revealed that 11 articles were not closely related to the subject in terms of purpose, method, and results; therefore, they were ruled out. A diagram of the study selection process is displayed in Figure 1 .

Flowchart of the literature review

For performing the study, one of the researchers was assigned to review the literature and check for the inclusion and exclusion criteria based on the title and abstract. After the removal of the articles that failed to pass the inclusion criteria, the full text of all the selected ones was investigated. Thereafter, the desired results were extracted considering a number of focused parameters and handed over to another researcher to review and revise them, if necessary. In general, out of 101 documents, 19 articles were investigated to extract the results.

In this study, independent variables included light characteristics (e.g., intensity, wavelength, and color temperature), environmental factor (e.g., noise, heat, and vibration), and personal factors (e.g., gender, sensitivity, and duration exposure), while attention and reaction time were regarded as dependent variables. Each of the reviewed articles examined several variables; therefore, the results of the reviewed articles were classified and evaluated in three main areas: (1) the effects of light-related factors on attention and reaction time ( Table 1 ); (2) the combined effects of light and other environmental factors on attention and reaction time ( Table 2 ); and (3) the effects of personal factors related to light sensitivity on attention and reaction time ( Table 3 ). Moreover, the effects of daylight and other comprehensive studies on cognitive processes, including attention and reaction time, were investigated.

The selected articles were published in Persian (n=5) and English (n=10) journals. They were generally experimental studies and encompassed various aspects of light and its effects on cognitive functions, including reaction time and attention. The list of final articles which met all the study criteria is presented in Tables 1 , ​ ,2 2 , ​ ,3 3 .

In the process of human-machine perception, cognitive activities, such as reaction time and attention, are considered structural elements and main cognitive reactions to external stimuli in order to understand and analyze the conditions of assigned task 19 , 20 . Human factor research on lighting has largely on light visual aspects, as well as visual disturbance and performance. Evidence on the light non-visual, psychological, and biological effects has recently been presented 7 . According to various studies reviewed in the present research, the effects of lighting on attention and reaction time can be analyzed as the following:

For indoor lighting, illuminance is one of the important factors which can indicate the quality of lighting conditions. In their study, Yang et al 21 indicated that illuminance significantly affected subjects’ attention and alertness (P<0.05). That is to say, higher illuminance leads to higher levels of alertness and attentiveness. The participants were most alert, least relaxed, and performed most concentrated under lighting conditions of 500lx. Another laboratory study illustrated that even in the absence of sleep and light deprivation, exposure to a higher illuminance at the eye level can induce subjective alertness and vitality, increase physiological arousal, and improve performance on a sustained attention task 22 . The same results were reported by Leichfried et al who concluded that early morning illumination improves subjective alertness and mood; nonetheless, it had no impact on melatonin level and mental performance of individuals 23 . Generally speaking, a high illuminance level could make subjects feel more alert and concentrated, pointing to its significance in the enhancement of people’s attention level.

Numerous studies have pointed to the positive effects of blue light on function and consciousness 24 , 25 . For instance, Motamedzadeh et al 26 demonstrated that compared to baseline conditions and 6500 K, blue-enriched white light (17000 K) effectively improved working memory and sustained attention of control room staff. The performance results of the participants in tasks requiring sustained attention in the mentioned study have also indicated that exposure to blue-enriched white light does not affect the error of action. Nevertheless, compared to baseline conditions, such exposure significantly reduced the number of deletions and response errors. Moreover, the results observed a significant difference between the mean removal error at 17000 K in blue-enriched white light and baseline conditions ( P =0.020).

In a laboratory study, Baek and Min 27 showed that exposure to blue-enriched white light after lunch reduced alpha-band activity and improved sustained attention. In the same context, Studer et al. 28 reported that participants demonstrated increased attention in two of the three attention-based tasks due to blue-enriched light, compared to red-enriched light in the morning (high illuminance about 1000 Lux and short duration less than 1 h). The results also indicated a reduction in reaction time in the performed tests. Along the same lines, Knaier et al 29 have found that differences in reaction time in the control group (compared to other participants) were 1 and 2 milliseconds (95% CI-9.5) for participants in white light and blue light conditions, respectively.

Bansal et al 30 also detected significant differences in screened factors, EEG delta/theta activity, mood, sustained attention (reaction time tasks), short-term memory (verbal memory task), and working memory (visual memory task) due to exposure to blue-enriched white light. The effects of blue-enriched white light on the improvement of cognitive function have also been reported in other studies, including Cajochen et al. 31 , Viola et al 24 , Vetter et al 32 , Kretschmer et al 33 , and Chellappa et al. 34 . Nonetheless, previous studies have proved that blue-enriched white light may lead to retinal damage and oxidative stress. Therefore, in order to reduce the negative effects of blue-enriched white light, its direct exposure is prevented 35 , 36 . Bozkurt et al 37 , in their study on the level of attention in two students with the lighting color change in the classroom, have found that the attention level in both subjects was higher in red light, as compared to that in white and green lights.

Some studies have addressed the effects of correlated color temperature (CCT) on visual function or its physiological and psychological effects 38 . Chellappa et al 39 have observed that fluorescent lamps with higher color temperatures could enhance consciousness, well-being, and visual comfort. Moreover, exposure to a color temperature of 2700 K led to a faster reaction time in tasks requiring sustained attention, as compared to a color temperature of 6500 K. Their results further have revealed that higher color temperature requires proportional attention in tasks due to its more melatonin suppression and faster reaction time.

Huang et al 40 have pointed to the effects of color temperature on focused and sustained attention under white LED desk lighting. In the stated study, three CCTs conditions (2700, 4300, and 6500 K) were evaluated, and the Chu attention test was used to measure focused and stable attention. A paired comparison of CCT conditions suggested that at 4300 K color temperature, the score was higher than other conditions. In other words, the focused and stable attention is higher at the mentioned color temperature. The abovementioned results are consistent with those reported by Yamagishi et al 41 who pointed to the effects of CCT conditions (8200, 5000, 5000, 2500 K: controlled lighting in 470lux) on young and old people using the NV (Night vision) test.

Studies in recent decades have assessed the effect of environmental factors on human comfort, health, and function. However, almost all of them considered the effect of only one factor, and there is a dearth of research on the combined effects of parameters in real conditions. Mohebbian et al. 20 have pointed out that in thermal comfort conditions (temperature 22°C), the increase of illuminance decreased reaction time and its error, indicating its positive effect on the reaction time. On the other hand, the results of the referred study demonstrated that increasing the temperature (37°C) and illuminance through increasing the reaction time and reaction time error of individuals can interfere with the cognitive process and reduce their performance.

Amiri et al 42 have indicated that although sound, heat, and light have no negative impact on physiological and cognitive function at their harmless and permissible levels, simultaneous exposure to their harmful levels in different combined conditions exerts a mutual impact on physiological and cognitive parameters (working memory and attention), acting independently with a separate mechanism or synergistically with a similar mechanism 42 . The interaction between light and heat, as well as their impact on cognitive functions, has also been shown in the study by Lucas et al 43 . Huang and Bavolar have also found that the combination of different environmental factors reduces performance. In other words, if their combined effects are more than those of their individual effect, they can intensify each other’s effects and have a similar effect mechanism; however, if the combined effects of environmental risk factors are equal to those of their individual effect, they will most likely have a separate effect 44 , 45 .

Hygge et al 46 have demonstrated that tasks were performed more rapidly but less accurately in the presence of the sound component. The stated study observed the significant association of sound, heat, and light with text reading and word recall. These interactions are indicating the theoretical possibility that sound, heat, and internal light directly affect cognitive processes without general or partial mediation, at least not in the way that the inverse U hypothesis suggests. In the same context, Monazzam et al 47 have observed that an increase in vibration acceleration significantly improved discomfort and heart rate; however, it did not affect the reaction time. The results of the referred study also suggested that vibration and illuminance did not have a significant combined effect on the variables of discomfort, heart rate, and reaction time.

Interpersonal sensitivity to light may affect consciousness, cognitive function, and sleep physiology differently 48 . The impact of light on a wide range of electrophysiological factors may vary from person to person. It has been specified that the function of the visual system is strongly influenced by gender differences 49 , 50 . Chellappa et al 51 have assessed the effect of gender differences on light perception, conscious attention, and sleep in humans. According to their results, in a task requiring sustained attention in blue-enriched white light compared to blue-free light, men had higher light perception and faster reaction time than women. The distribution of Psychomotor Vigilance Task (PVT) reaction times (the number of RT observations between 500-100 ms) illustrated that light at 6500 K color temperature led to the movement toward a faster RT range than light at 2500 K color temperature for men.

Sunlight seems to be the most effective among various light sources since it contains a sufficient amount and a wide range of light. Natural light, due to its role in the production of vitamin D in human blood, can improve mental mood, attention, cognitive function, physical activity, sleep quality, and consciousness 52 . In their study, Shishegar et al 53 analyzed the effects of daylighting on the health and consciousness of workers and students. As illustrated by their results, the health, satisfaction, attention, and performance of workers and students are improved by natural light 52 .

Furthermore, Sahin et al 54 studies daylight exposure and its impacts on biomarkers, consciousness, and performance. They classified 13 subjects in illuminance ((low light (<5 lux), red light (max = 631 nm, 213 lux, 1.1W/m2), and white light (2568K, 361 lux, 1.1W /m2) conditions. The results of the mentioned study indicated that red light could increase the short-term performance, reduce reaction time significantly (P=0.05), and improve power in functional tests during a day. They also confirmed the hypothesis that exposure to daylight at long and narrow (red) or polychromatic wavelengths (2568 K) causes higher consciousness and shorter reaction time. The abovementioned results have been reported in similar studies performed by Figueiro et al. 55 and Lafrance et al 56 .

Reaction time is the very short time that elapses between the presentation of a stimulus and the recording of the subject response. In healthy individuals, it usually lasts from 10-12 cent seconds, appearing voluntary and reflectively 57 . In other words, reaction time is the elapsed time for a person to understand the situation and process a response 58 . Smolders et al. 11 have examined a mixed group of individuals (n=32) in different blocks through functional tests at two levels of illuminance (200lux or 1000lux at the eye level, 4000 K) for one hour in the morning and one hour in the afternoon. The results of the stated study showed that an increase in the illuminance (1000 lux, compared to 200lux) led to improved cognitive function, enhanced consciousness, less somnolence, more energy, and shorter reaction time.

Dehghan et al 59 indicated that after 90 min of exposure, simple, diagnostic, dichromatic selection, and two-tone selection reaction time were significant at all lighting levels (P<0.0001). According to the results of the referred study, after 90 min of exposure, the minimum and the maximum reaction time scores in illuminance were 200 lux and 1500 lux, respectively. In addition, the maximum and minimum response errors were at the level of 200 lux (0.5) and 500 lux (0.1), respectively. The removal response was also significant at different levels of illuminance (P=0.017); therefore, the maximum and minimum removal responses were reported at the level of 200 and 1500 lux (0.1), respectively. Chang (2013) has achieved similar results regarding the effects of light on attention and reaction time using the PVT test (psychomotor vigilance task) in people exposed to 1 lux illuminance, as compared to 90 lux 60 . Correa et al. have found that blue-enrich white light led to a greater improvement in reaction time with higher levels of baseline consciousness 61 .

"Attention" is a cognitive process defined as a selective focus on one aspect of the environment while ignoring others. It is also attributed to the allocation of resource processing 13 . The word "attention" can be defined in accordance with the number of errors made during a test; therefore, more assiduous attention during the test leads to fewer errors and vice versa 62 . An increase in attention function has been reported after 6.5 h of light exposure with short wavelengths (460 nm), compared to those with long wavelengths (550.55 nm) 63 . Dehghan et al 59 investigated the effects of different levels of illuminance on the rate of attention and reaction time in laboratory conditions. The findings of the stated study indicated that the maximum and minimum percentages of attention in 1500 and 500 lux illuminance equal 99.75% and 99.36%, respectively.

This finding is in line with that obtained by Smolders et al 11 who revealed that increasing illuminance improved cognitive functions in individuals. Amiri et al. 42 also pointed out that although the mean scores of working memory and attention in low light level exposure are lower than harmless level exposure, this difference is not statistically significant. A field study conducted in schools has demonstrated that classroom lighting with variable illuminance ("focused" program: very bright, cold light: 1060 lux) enhanced attention in students 64 . The majority of studies reported that higher illuminance (1000-5000 lux vs. 5-200 lux for 1-5 h) is associated with increased function and attention 65 , 66 . Kretschmer suggested that exposure to bright light at night reduced error rates in tasks requiring memory and focused performance; nonetheless, it was is completely ineffective in tasks requiring sustained attention 33 .

As evidenced by the results of the present study, it can be stated that lighting affects the attention and reaction time; therefore, it should be designed to meet non-visual needs, apart from comfort and visual requirements. The parameters of the wavelength, color temperature, and intensity of light modulate the brain responses, including attention and reaction time. The best light in the regulation of psychological, biological, and cognitive processes is bright daylight in the morning with a short wavelength, high intensity, as well as stronger and more lasting effects. Shorter wavelengths, compared to the longer ones, lead to suppressed Melatonin, higher consciousness, less somnolence, increased attention function, and faster reaction time. When exposed to monochromatic light, non-visual responses are most sensitive to blue light (wavelengths between 459 and 483 nm).

The effects of blue light on the enhancement of cognitive function have been reported in various studies; nonetheless, direct exposure to it may cause retinal damage and oxidative stress. Illuminance, in addition to wavelength, affects reaction time and attention. The majority of studies have reported higher illuminance to be associated with increased consciousness, decreased somnolence, increased attention, and faster reaction time. Moreover, the assessment of the effects of color temperature demonstrated that higher color temperature is associated with greater melatonin suppression and faster reaction time in tasks that require attention. Therefore, the design and use of light in workplaces should be performed to meet non-visual and cognitive needs, such as attention and reaction time, in addition to providing comfort and visual needs. The effects of light on attention and reaction time can be presented through the following conceptual model. ( Figure 2 )

Conceptual model for the effects of light on attention and reaction time

Acknowledgments

The authors would like to thank the Hamadan University of Medical Sciences for the library support of this study.

Conflict of interests

The authors declare that they have no conflict of interest.

Not funding.

• The light is a powerful modulator of non-visual performance in cognitive tasks.
• The light with shorter wavelengths, higher intensity, and higher color temperature lead to increased attention and faster reaction time.
• The best light in the regulation of psychological, biological, and cognitive processes is bright daylight in the morning with a short wavelength and high intensity.
• Simultaneous exposure to harmful levels of environmental factors interacts with cognitive and physiological parameters.

Citation: Golmohammadi R, Yousefi H, Safarpour Khotbesara N, Nasrolahi A, Kurd N. Effects of Light on Attention and Reaction Time: A Systematic Review. J Res Health Sci. 2021; 21(4): e00529.

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Physicists create new form of light

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Try a quick experiment: Take two flashlights into a dark room and shine them so that their light beams cross. Notice anything peculiar? The rather anticlimactic answer is, probably not. That’s because the individual photons that make up light do not interact. Instead, they simply pass each other by, like indifferent spirits in the night.

But what if light particles could be made to interact, attracting and repelling each other like atoms in ordinary matter? One tantalizing, albeit sci-fi possibility: light sabers — beams of light that can pull and push on each other, making for dazzling, epic confrontations. Or, in a more likely scenario, two beams of light could meet and merge into one single, luminous stream.

It may seem like such optical behavior would require bending the rules of physics, but in fact, scientists at MIT, Harvard University, and elsewhere have now demonstrated that photons can indeed be made to interact — an accomplishment that could open a path toward using photons in quantum computing, if not in light sabers.

In a paper published today in the journal Science , the team, led by Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, and Professor Mikhail Lukin from Harvard University, reports that it has observed groups of three photons interacting and, in effect, sticking together to form a completely new kind of photonic matter.

In controlled experiments, the researchers found that when they shone a very weak laser beam through a dense cloud of ultracold rubidium atoms, rather than exiting the cloud as single, randomly spaced photons, the photons bound together in pairs or triplets, suggesting some kind of interaction — in this case, attraction — taking place among them.

While photons normally have no mass and travel at 300,000 kilometers per second (the speed of light), the researchers found that the bound photons actually acquired a fraction of an electron’s mass. These newly weighed-down light particles were also relatively sluggish, traveling about 100,000 times slower than normal noninteracting photons.

Vuletic says the results demonstrate that photons can indeed attract, or entangle each other. If they can be made to interact in other ways, photons may be harnessed to perform extremely fast, incredibly complex quantum computations.

“The interaction of individual photons has been a very long dream for decades,” Vuletic says.

Vuletic’s co-authors include Qi-Yung Liang, Sergio Cantu, and Travis Nicholson from MIT, Lukin and Aditya Venkatramani of Harvard, Michael Gullans and Alexey Gorshkov of the University of Maryland, Jeff Thompson from Princeton University, and Cheng Ching of the University of Chicago.

Biggering and biggering

Vuletic and Lukin lead the MIT-Harvard Center for Ultracold Atoms, and together they have been looking for ways, both theoretical and experimental, to encourage interactions between photons. In 2013, the effort paid off, as the team observed pairs of photons interacting and binding together for the first time, creating an entirely new state of matter.

In their new work, the researchers wondered whether interactions could take place between not only two photons, but more.

“For example, you can combine oxygen molecules to form O 2 and O 3 (ozone), but not O 4 , and for some molecules you can’t form even a three-particle molecule,” Vuletic says. “So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?”

To find out, the team used the same experimental approach they used to observe two-photon interactions. The process begins with cooling a cloud of rubidium atoms to ultracold temperatures, just a millionth of a degree above absolute zero. Cooling the atoms slows them to a near standstill. Through this cloud of immobilized atoms, the researchers then shine a very weak laser beam — so weak, in fact, that only a handful of photons travel through the cloud at any one time.

The researchers then measure the photons as they come out the other side of the atom cloud. In the new experiment, they found that the photons streamed out as pairs and triplets, rather than exiting the cloud at random intervals, as single photons having nothing to do with each other.

In addition to tracking the number and rate of photons, the team measured the phase of photons, before and after traveling through the atom cloud. A photon’s phase indicates its frequency of oscillation.

“The phase tells you how strongly they’re interacting, and the larger the phase, the stronger they are bound together,” Venkatramani explains. The team observed that as three-photon particles exited the atom cloud simultaneously, their phase was shifted compared to what it was when the photons didn’t interact at all, and was three times larger than the phase shift of two-photon molecules. “This means these photons are not just each of them independently interacting, but they’re all together interacting strongly.”

Memorable encounters

The researchers then developed a hypothesis to explain what might have caused the photons to interact in the first place. Their model, based on physical principles, puts forth the following scenario: As a single photon moves through the cloud of rubidium atoms, it briefly lands on a nearby atom before skipping to another atom, like a bee flitting between flowers, until it reaches the other end.

If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.

“What was interesting was that these triplets formed at all,” Vuletic says. “It was also not known whether they would be equally, less, or more strongly bound compared with photon pairs.”

The entire interaction within the atom cloud occurs over a millionth of a second. And it is this interaction that triggers photons to remain bound together, even after they’ve left the cloud.

“What’s neat about this is, when photons go through the medium, anything that happens in the medium, they ‘remember’ when they get out,” Cantu says.

This means that photons that have interacted with each other, in this case through an attraction between them, can be thought of as strongly correlated, or entangled — a key property for any quantum computing bit.

“Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibers,” Vuletic says. “If photons can influence one another, then if you can entangle these photons, and we’ve done that, you can use them to distribute quantum information in an interesting and useful way.”

Going forward, the team will look for ways to coerce other interactions such as repulsion, where photons may scatter off each other like billiard balls.

“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic says. “With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It’s very uncharted territory.”

This research was supported in part by the National Science Foundation.

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Press mentions, new scientist.

Research by Physics PhD candidate Sergio Cantu has led to the discovery of a new form of light, which happens when photos stick together, as opposed to passing through one another. “’We send the light into the medium, it gets effectively dressed up as if it were atoms, and then when it turns back into photons they remember interactions that happened in the medium,” Cantu explains to Leah Crane at New Scientist . 

Writing for Newsweek, Katherine Hignett reports that for the first time, scientists have observed groups of three photons interacting and effectively producing a new form of light. “Light,” Prof. Vladan Vuletic, who led the research, tells Hignett, “is already used to transmit data very quickly over long distances via fiber optic cables. Being able to manipulate these photons could enable the distribution of data in much more powerful ways.”

Motherboard

MIT physicists have created a new form of light that allows up to three photons to bind together, writes Daniel Oberhaus for Motherboard . While the research is experimental, Oberhaus writes that the trio of photons “are much more strongly bound together and are, as a result, better carriers of information” than other photonic qubits.

Smithsonian Magazine

Research published in Science demonstrates the ability of photons to bind together in a way previously thought impossible – creating a new form of light. “The photon dance happens in a lab at MIT where the physicists run table-top experiments with lasers,” writes Marissa Fessenden for Smithsonian . “Photons bound together in this way can carry information – a quality that is useful for quantum computing.”

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Light Pollution

People all over the world are living under the nighttime glow of artificial light, and it is causing big problems for humans, wildlife, and the environment. There is a global movement to reduce light pollution, and everyone can help.

Conservation, Earth Science, Astronomy

Hong Kong Light Pollution

Boats, buildings, street lights, and even fireworks contribute to the light pollution in Victoria Harbor, Hong Kong. Light pollution can be detrimental to the health of people and animals in the area.

Photograph by Jodi Cobb

Most environmental pollution on Earth comes from humans and their inventions. Take, for example, the automobile or that miraculous human-made material, plastic . Today, automobile emissions are a major source of air pollution contributing to climate change, and plastics fill our ocean, creating a significant health hazard to marine animals.

And what about the electric lightbulb, thought to be one of the greatest human inventions of all time? Electric light can be a beautiful thing, guiding us home when the sun goes down, keeping us safe and making our homes cozy and bright. However, like carbon dioxide emissions and plastic , too much of a good thing has started to negatively impact the environment. Light pollution , the excessive or inappropriate use of outdoor artificial light, is affecting human health, wildlife behavior, and our ability to observe stars and other celestial objects.

That Earthly Sky Glow

Light pollution is a global issue. This became glaringly obvious when the World Atlas of Night Sky Brightness , a computer-generated map based on thousands of satellite photos, was published in 2016. Available online for viewing, the atlas shows how and where our globe is lit up at night. Vast areas of North America, Europe, the Middle East, and Asia are glowing with light, while only the most remote regions on Earth (Siberia, the Sahara, and the Amazon) are in total darkness. Some of the most light-polluted countries in the world are Singapore, Qatar, and Kuwait.

Sky glow is the brightening of the night sky, mostly over urban areas, due to the electric lights of cars, streetlamps, offices, factories, outdoor advertising, and buildings, turning night into day for people who work and play long after sunset.

People living in cities with high levels of sky glow have a hard time seeing more than a handful of stars at night. Astronomers are particularly concerned with sky glow pollution as it reduces their ability to view celestial objects.

More than 80 percent of the world’s population, and 99 percent of Americans and Europeans, live under sky glow. It sounds pretty, but sky glow caused by anthropogenic activities is one of the most pervasive forms of light pollution .

Is it Time to Get Up?

Artificial light can wreak havoc on natural body rhythms in both humans and animals. Nocturnal light interrupts sleep and confuses the circadian rhythm—the internal, twenty-four-hour clock that guides day and night activities and affects physiological processes in nearly all living organisms. One of these processes is the production of the hormone melatonin , which is released when it is dark and is inhibited when there is light present. An increased amount of light at night lowers melatonin production, which results in sleep deprivation, fatigue, headaches, stress, anxiety, and other health problems. Recent studies also show a connection between reduced melatonin levels and cancer. In fact, new scientific discoveries about the health effects of artificial light have convinced the American Medical Association (AMA) to support efforts to control light pollution and conduct research on the potential risks of exposure to light at night. Blue light, in particular, has been shown to reduce levels of melatonin in humans. Blue light is found in cell phones and other computer devices, as well as in light-emitting diodes (LEDs), the kinds of bulbs that have become popular at home and in industrial and city lighting due to their low cost and energy efficiency.

Animals are Lost and Confused, Too

Studies show that light pollution is also impacting animal behaviors, such as migration patterns , wake-sleep habits, and habitat formation. Because of light pollution , sea turtles and birds guided by moonlight during migration get confused, lose their way, and often die. Large numbers of insects, a primary food source for birds and other animals, are drawn to artificial lights and are instantly killed upon contact with light sources. Birds are also affected by this, and many cities have adopted a “Lights Out” program to turn off building lights during bird migration.

A study of blackbirds ( Turdus merula)  in Germany found that traffic noise and artificial night lighting causes birds in the city to become active earlier than birds in natural areas—waking and singing as much as five hours sooner than their country cousins. Even animals living under the sea may be affected by underwater artificial lighting. One study looked at how marine animals responded to brightly lit panels submerged under water off the coast of Wales. Fewer filter feeding animals, such as the sea squirt and sea bristle, made their homes near the lighted panels. This could mean that the light from oil rigs, passing ships, and harbors is altering marine ecosystems .

Even in places meant to provide protected natural habitats for wildlife, light pollution is making an impact. The National Park Service (NPS) has made maintaining a dark night sky a priority. The NPS Night Skies Team has been monitoring night sky brightness in some one hundred parks, and nearly every park showed at least some light pollution.

You Shouldn’t Need Sunglasses at Night

There are three other kinds of light pollution: glare, clutter, and light trespass. Glare is excessive brightness that can cause visual discomfort (for example, when driving). Clutter is bright, confusing, and excessive groupings of light sources (for example, Times Square in New York City, New York). Light trespass is when light extends into an area where it is not wanted or needed (like a streetlight illuminating a nearby bedroom window). Most outdoor lighting is poorly positioned, sending wasted electricity up into the sky.

Bring Back the Dark Sky

There are several organizations working to reduce light pollution. One of these is the U.S.-based International Dark Sky Association (IDA), formed in 1988 to preserve the natural night sky. IDA educates the public and certifies parks and other places that have worked to reduce their light emissions. In 2017, the IDA approved the first U.S. dark sky reserve. The massive Central Idaho Dark Sky Reserve, which clocks in at 3,667 square kilometers (1,416 square miles), joined eleven other dark sky reserves established around the world. As of December of 2018, IDA lists thirteen dark sky reserves on their site.

Stop Wasting Energy: Things We Can All Do

More people are taking action to reduce light pollution and bring back the natural night sky. Many states have adopted legislation to control outdoor lighting, and manufacturers have designed and produced high-efficiency light sources that save energy and reduce light pollution.

Individuals are urged to use outdoor lighting only when and where it is needed, to make sure outdoor lights are properly shielded and directing light down instead of up into the sky, and to close window blinds, shades, and curtains at night to keep light inside.

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Lyndie Chiou

The Hunt for Ultralight Dark Matter

The original version of this story appeared in Quanta Magazine .

The end is brutal for electrons hurtling at 99.9999999 percent of the speed of light through SLAC National Accelerator Laboratory’s two-mile-long beam pipe: a final slam into End Station A. In the late 1960s and early ’70s, such collisions broke apart protons and neutrons to reveal the elementary particles that make them up. The discovery won the experiment’s leader a Nobel Prize. “End Station A is this hallowed ground at SLAC,” said the physicist Timothy Nelson .

Walking to the back of the warehouse, past piles of equipment, Nelson pointed at the skeleton of an old experiment, beyond the point where the tree-trunk-sized pipe of the historic accelerator cut off. It’s there, he said, that a soon-to-be-constructed experiment will see—or quickly rule out—one of the most popular new candidates for dark matter.

Almost a century ago, the Swiss astrophysicist Fritz Zwicky described a galaxy cluster that appeared to rotate too fast to be held together by its visible mass. He proposed that invisible matter was lending its gravity to the situation. The evidence has grown and grown, and researchers now believe that 85 percent of the universe’s matter is hidden. But the mystery of dark matter’s identity has endured.

For decades, researchers focused on two candidate particle types: weakly interacting massive particles (WIMPs) and axions. These are the simplest formulations for dark matter, and each type of particle would also elegantly solve other physics mysteries. But after roughly 40 years of vain searches for these particles—searches that have almost entirely ruled out the chance that dark matter is made of ordinary WIMPs—physicists have become far more open-minded about what dark matter might be. Maybe dark matter is not simple at all. It could, some suggest, comprise a whole family of particles, as does visible matter.

“The most common hypothesis is that this is somehow simple. Why on earth should we expect that?” said Philip Schuster , a theoretical physicist at Stanford University. “That is not what nature has been trying to tell us for the past 200 years.”

Move away from the WIMP or axion paradigms, and you probably have to give up on the idea that dark matter consists of a single type of particle. Instead, new models involve a bevy of extraordinarily lightweight entities, sometimes called feebly interacting particles. Today’s dark matter hunters are especially excited by two categories of these particles: “light dark matter” and “ultralight dark matter.” Light dark matter can be thought of as lighter variations on WIMPs, while ultralight dark matter is made of even lighter axions.

End Station A is the site of an experiment at SLAC National Accelerator Laboratory where quarks were discovered more than half a century ago. Now SLAC physicists including Tim Nelson (right) are poised to build a new experiment here to look for dark matter.

By Joseph Cox

By Matt Burgess

By Marah Eakin

With whole families of lighter-weight, more feebly interacting dark matter candidates being floated, multitudes of smaller, faster experiments are being developed all over the world to look for them.

In 2019, the Department of Energy launched its Dark Matter New Initiatives program to fund research into experiments that could reach their conclusions quickly—in years instead of the decades required by traditional dark matter detection efforts. Today, a number of these projects are ready to begin construction. Experiments targeting different chunks of the space of possibilities are also under development in Europe and Asia.

“If you go back to the way particle physics used to be back in the 1920s and ’30s, it was all small-scale efforts, often by universities or private donors, who would support these little efforts that ended up revolutionizing the field,” Schuster said. “And coming out of the Second World War, the operation shifted toward large-scale, big projects. Small experimental efforts kind of just withered away.”

Schuster and Natalia Toro , also a Stanford physicist, are part of the effort to find the needle—or needles—inside the haystack of hidden particle possibilities.

“There’s this lower-mass range of dark matter that has very similar physics,” Toro said. “Very theoretically reasonable and accessible experimentally. We just haven’t tried turning over those stones. We probably should.”

WIMPs and Axions

WIMPs, which could range from about one to 100,000 times the mass of a proton, would match up neatly with a primordial origin story. They have the correct masses and interaction strengths to have formed abundantly during the Big Bang before annihilating each other at the right rate to become the leftover dark matter inferred in the universe today, a so-called thermal relic.

WIMPs also became a dark matter darling because of the “WIMP miracle”: Their hypothesized mass range and interactions fit the profile of one of the particles predicted by supersymmetry, an appealing hypothetical extension of the known elementary particles.

Since the 1990s, experiments have been looking for signs of WIMPs occasionally interacting with protons and neutrons via the weak force. But these searches have come up empty. This means WIMPs would have to interact via the weak force even more feebly than expected. At some point, their interaction strength becomes too low to match the thermal-relic model.

While the search for WIMPs isn’t over, there are not many places left to look. Detectors have become so effective at finding weakly interacting bits of matter that they’re effectively blinded by all the neutrinos coming from the sun, Toro said. “So that’s kind of the end of the WIMP detection era.”

Axions, the other early favorite, are so lightweight that they behave more like waves than particles, similar to our understanding of massless particles such as photons. These candidate particles achieve their own particle-physics miracle, solving the “strong CP” problem. In brief, the laws of particle physics do not explain why the strong force—which holds together atomic nuclei—behaves the same when the charge and “parity” of particles are inverted. This “CP symmetry” points to missing physics. Axions were introduced as a way to ensure symmetry via a tiny force field that stabilizes the strong interactions.

For decades, experiments have searched for axions, albeit at a snail’s pace. The first-generation detectors needed to take long measurements to distinguish between noise and the muffled signal of potential axion interactions. As a result, only a small slice of the axion mass range has been explored. But so far, no axions.

Lighter Dark Matter

By around 2008, the notion of a dark matter particle similar to a WIMP, but smaller, began to emerge. For physicists, this “light dark matter” is akin to seeing a familiar face in a foreign land: The proposed particles exist in a range of masses similar to those of ordinary matter—electron- to proton-size. (A proton weighs the same as about 1,800 electrons.)

But once a feebly interacting particle’s mass drops below about the mass of a proton, it couldn’t have collided and annihilated so easily in the early universe. So, to match the thermal relic, light dark matter requires the existence of at least one unknown force that would affect the dark matter particles.

Every force of nature is mediated by a force-carrying particle. The particle that mediates the hypothetical force associated with light dark matter falls in a class of “portal particles” that bridge the gap between ordinary matter and dark matter. According to Toro, “light” portal particles would interact with light dark matter and very weakly with ordinary matter. One popular candidate is a dark photon, a direct analogue of the familiar photon. Other portals are possible too, such as Higgs-like particles or force carriers that have no known analogue.

The SLAC and Stanford University particle physicists Natalia Toro and Philip Schuster have been leaders in the effort to dream up new possibilities for the nature of dark matter and how to look for them.

If or when SLAC’s planned project, the Light Dark Matter Experiment (LDMX), receives funding—a decision from the Department of Energy is expected in the next year or so—it will scan for light dark matter. The experiment is designed to accelerate electrons toward a target made of tungsten in End Station A. In the vast majority of collisions between a speeding electron and a tungsten nucleus, nothing interesting will happen. But rarely—on the order of once every 10,000 trillion hits, if light dark matter exists—the electron will instead interact with the nucleus via the unknown dark force to produce light dark matter, significantly draining the electron’s energy.

That 10,000 trillion is actually the worst-case scenario for light dark matter. It’s the lowest rate at which you can produce dark matter to match thermal-relic measurements. But Schuster says light dark matter might arise in upward of one in every 100 billion impacts. If so, then with the planned collision rate of the experiment, “that’s an inordinate amount of dark matter that you can produce.”

LDMX will need to run for three to five years, Nelson said, to definitively detect or rule out thermal relic light dark matter.

Ultralight Dark Matter

Other dark matter hunters have their experiments tuned for a different candidate. Ultralight dark matter is axionlike but no longer obliged to solve the strong CP problem. Because of this, it can be much more lightweight than ordinary axions, as light as 10 billionths of a trillionth of the electron’s mass. That tiny mass corresponds to a wave with a vast wavelength, as long as a small galaxy. In fact, the mass can’t be any smaller because if it were, the even longer wavelengths would mean that dark matter could not be concentrated around galaxies, as astronomers observe.

Ultralight dark matter is so incredibly minuscule that the dark-force particle needed to mediate its interactions is thought to be massive. “There’s no name given to these mediators,” Schuster said, “because it’s outside of any possible experiment. It has to be there [in the theory] for consistency, but we don’t worry about them.”

The origin story for ultralight dark matter particles depends on the particular theoretical model, but Toro says they would have arisen after the Big Bang, so the thermal-relic argument is irrelevant. There’s a different motivation for thinking about them. The particles naturally follow from string theory, a candidate for the fundamental theory of physics. These feeble particles arise from the ways that six tiny dimensions might be curled up or “compactified” at each point in our 4D universe, according to string theory. “The existence of light axionlike particles is strongly motivated by many kinds of string compactifications,” said Jessie Shelton, a physicist at the University of Illinois, “and it’s something that we should take seriously.”

Rather than trying to create dark matter using an accelerator, experiments looking for axions and ultralight dark matter listen for the dark matter that supposedly surrounds us. Based on its gravitational effects, dark matter seems to be distributed most densely near the Milky Way’s center, but one estimate suggests that even out here on Earth, we can expect dark matter to have a density of almost half a proton’s mass per cubic centimeter. Experiments try to detect this ever-present dark matter using powerful magnetic fields. In theory, the ethereal dark matter will occasionally absorb a photon from the strong magnetic field and convert it into a microwave photon, which an experiment can detect.

Two proposed experiments intend to pursue the dark frequencies: Stanford University’s DM Radio and the ADMX-EFR, or Axion Dark Matter Experiment Extended Frequency Range, at the University of Washington. Both experiments were originally conceived to search for axions, but are now being revised to scan for the lighter variants.

The biggest challenge for both experiments—which also await the Department of Energy’s funding decisions—lies in the faintness of the predicted microwave photons. The tiny signal requires all experimental noise to be removed other than the system’s inherent quantum jitter, a hurdle that researchers think they can overcome.

While these researchers await the faint signals of feeble particles, other dark matter ideas remain in play. Some theoretical physicists are reconsidering a long-sidelined idea that this invisible matter might take the form of primordial black holes created during the Big Bang. One more possibility is that the prevailing theory of gravity isn’t quite right . So far, however, competing gravity theories haven’t garnered much interest.

“At this point,” Schuster said, “let’s be honest, everybody is guessing.”

Original story reprinted with permission from Quanta Magazine , an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.

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Nanoscale engineering brings light-twisting materials to more extreme settings

• Kate McAlpine

New manufacturing method builds tougher materials that were previously considered useless for twisting light into more robust optical devices

Imaging the hot turbulence of aircraft propulsion systems may now be possible with sturdy sheets of composite materials that twist light beams, according to research led by the University of Michigan and Air Force Research Laboratory.

The sheets were produced with a new manufacturing method that opens possibilities beyond aircraft design, as it enables new classes of materials to be used in polarization optics. While the team demonstrated high temperature tolerance, new mechanical, electrical and physical properties are expected to emerge as well—with potential applications in energy, sensors for vehicles and robots, and space exploration.

“Combining multiple functionalities into 2D materials opens up a world of possibilities,” said Dhriti Nepal , senior research materials engineer at the Air Force Research Laboratory and a co-corresponding author of the study published recently in Nature.

“Think of a butterfly’s wings, which allow it to fly, regulate temperature, and reflect light to produce specific colors for attracting mates and avoiding predators. This technique provides new design opportunities for creating multifunctional devices capable of anything one can imagine.”

The key is arranging nanomaterials that don’t twist light on their own onto layers that turn light waves into either left- or right-handed spirals, known as circular polarizations. In the aircraft example, turbulence created by the engine spins the light, which is then filtered through the material for imaging. Today, devices like LCD screens and thermochromic paints already control the twist and orientation of light waves using liquid crystals , but they melt not far above ambient temperatures.

“There could be situations in which you want to twist light outside the normal operating temperatures of liquid crystals. Now, we can make light-polarizing devices for those kinds of settings,” said Nicholas Kotov , the Irving Langmuir Distinguished University Professor of Chemical Sciences and Engineering at U-M and lead author of the study.

The new material can twist light at 250 degrees Celsius, and through the imaging of turbulence in aircraft engines and other applications, it could enable aerospace engineers to improve designs for better aircraft flight performance.

“Future aerospace systems continue to push the edge of technical feasibility. These low-cost optical materials afford modularity, which is crucial for optimizing solutions for a broad range of future technologies,” said Richard Vaia , materials and manufacturing chief scientist at the Air Force Research Laboratory and a corresponding author of the study.

To make the materials, the researchers put microscopic grooves into a plastic sheet and covered it with several layers of tiny, flat particles with a diameter 10,000 times smaller than a millimeter. These particles were held in place with alternating layers of a molecular adhesive, and they could be made from any material that can be made into flat nanoparticles. For their heat-tolerant materials, the researchers used ceramic-like materials called MXenes.

As light moves through the material, it divides into two beams, one with horizontally oscillating waves and another with vertically oscillating waves. The vertical waves pass through faster than the horizontal waves. As a result, the waves exit out of phase and appear as a spiral of light. The angle of the grooves determines the direction in which the light spirals, and layers of silver nanowires can help ensure the light spirals solely to the left or right.

“Our calculations suggest that the optical properties didn’t come from the nanoplates themselves, but from their orientation on the grooves caused by our fabrication process,” said André Farias de Moura, associate professor of chemistry at the Federal University of São Carlos and a co-corresponding author of the study.

The research was funded by the National Science Foundation Science and Technology Center for Complex Particle Systems , U.S. Department of Defense, U.S. Office of Naval Research, U.S. Air Force Office of Scientific Research, Brazil’s National Council for Scientific and Technological Development, as well as the São Paulo Research Foundation. The materials were studied at the Michigan Center for Materials Characterization .

Felippe Colombari from the Brazilian Biorenewables National Laboratory also contributed to the study. Nicholas Kotov is also the Joseph B. and Florence V. Cejka Professor of Engineering and a professor of macromolecular science and engineering.

Kotov filed a patent with the help of Innovation Partnerships at U-M, and his company Photon Semantics LLC, a U-M startup, is planning to license the technology. Kotov and the University of Michigan have a financial interest in Photon Semantics LLC.

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Researchers Create Materials With Unique Combo of Stiffness, Thermal Insulation

For Immediate Release

Researchers have demonstrated the ability to engineer materials that are both stiff and capable of insulating against heat. This combination of properties is extremely unusual and holds promise for a range of applications, such as the development of new thermal insulation coatings for electronic devices.

“Materials that have a high elastic modulus tend to also be highly thermally conductive, and vice versa,” says Jun Liu, co-corresponding author of a paper on the work and an associate professor of mechanical and aerospace engineering at North Carolina State University. “In other words, if a material is stiff, it does a good job of conducting heat. And if a material is not stiff, then it is usually good at insulating against heat.

“But there are instances where you’d want materials that are stiff, but are also good insulators,” Liu says. “For example, you might want to create thermal insulation coatings to protect electronics from high temperatures. Historically, that’s been a challenge.

“We’ve now discovered a range of materials that are both stiff and excellent thermal insulators. What’s more, we can engineer the materials as needed to control how stiff they are and how thermally conductive they are.”

Specifically, the researchers were working with a subset of the class of materials called two-dimensional hybrid organic-inorganic perovskites (2D HOIP).

“These are thin films consisting of alternating organic and inorganic layers in a highly ordered crystalline structure,” says Wei You, co-corresponding author of this paper and professor of chemistry and applied physical sciences at the University of North Carolina at Chapel Hill. “And we can tune the composition of either the inorganic or organic layer.”

“We found that we can control the elastic modulus and thermal conductivity of some 2D HOIPs by replacing some of the carbon-carbon chains in the organic layers with benzene rings,” says Qing Tu, co-corresponding author of this paper and an assistant professor of materials science and engineering at Texas A&M University. “Basically – within this specific subset of layered materials – the more benzene rings we add, the stiffer the material gets, and the better able it is to insulate against heat.”

“While discovering these materials in itself holds tremendous potential for a range of applications, as researchers we are particularly excited because we’ve identified the mechanism that is responsible for these characteristics – namely the critical role that the benzene rings play,” says Liu.

In experiments, the researchers found at least three distinct 2D HOIP materials that became less thermally conductive the stiffer they got.

“This work is exciting because it suggests a new pathway for engineering materials with desirable combinations of properties,” Liu says.

The researchers also discovered another interesting phenomenon with 2D HOIP materials. Specifically, they found that by introducing chirality into the organic layers – i.e., making the carbon chains in those layers asymmetrical – they could effectively maintain the same stiffness and thermal conductivity even when making substantial changes to the composition of the organic layers.

“This raises some interesting questions about whether we might be able to optimize other characteristics of these materials without having to worry about how those changes might influence the material’s stiffness or thermal conductivity,” says Liu.

The paper, “ Anomalous correlation between thermal conductivity and elastic modulus in two-dimensional hybrid metal halide perovskites ,” is published in the journal ACS Nano . Ankit Negi, a former Ph.D. student at NC State, is first author of the paper. Co-authors include Cong Yang, Andrew Comstock, Saqlain Raza and Ziqi Wang, Ph.D. students at NC State; Subhrangsu Mukherjee, a former Ph.D. student at NC State; Dali Sun, an associate professor of physics at NC State; Harald Ade, Goodnight Innovation Distinguished Professor of Physics at NC State; Liang Yan of UNC; and Yeonju Yu and Doyun Kim of Texas A&M.

The work was done with support from the National Science Foundation, under grants 1943813, 2311573 and 2154791; the Office of Naval Research, under grant N000142012155; and the Department of Energy, under grant DE-SC0020992.

Note to Editors: The study abstract follows.

“Anomalous correlation between thermal conductivity and elastic modulus in two-dimensional hybrid metal halide perovskites”

Authors : Ankit Negi, Cong Yang, Subhrangsu Mukherjee, Andrew H. Comstock, Saqlain Raza, Ziqi Wang, Dali Sun, Harald Ade, and Jun Liu, North Carolina State University; Liang Yan and Wei You, University of North Carolina at Chapel Hill; Yeonju Yu, Doyun Kim and Qing Tu, Texas A&M University

Published : May 24, ACS Nano

DOI : 10.1021/acsnano.3c12172

Abstract: Device-level implementation of soft materials for energy conversion and thermal management demands a comprehensive understanding of their thermal conductivity and elastic modulus to mitigate thermo-mechanical challenges and ensure long-term stability. Thermal conductivity and elastic modulus are usually positively correlated in soft materials, such as amorphous macromolecules, which poses a challenge to discover materials that are either soft and thermally conductive or hard and thermally insulative. Here, we show anomalous correlations of thermal conductivity and elastic modulus in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIP) by engineering the molecular interaction between organic cations. By replacing conventional alkyl-alkyl and aryl-aryl type organic interactions with mixed alkyl-aryl ones, we observe enhancement in elastic modulus with a reduction in thermal conductivity. This anomalous dependence provides a route to engineer thermal conductivity and elastic modulus independently and a guideline to search for better thermal management materials. Further, introducing chirality into the organic cation induces a molecular packing that leads to the same thermal conductivity and elastic modulus regardless of the composition across all half-chiral 2D HOIPs. This finding provides substantial leeway for further investigations in chiral 2D HOIPs to tune optoelectronic properties without compromising the thermal and mechanical stability.

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Eclipsing the Limitations of Solar Energy

Uc davis researchers are innovating a bright future for clean energy by boosting solar panel efficiency and making around-the-clock solar energy a reality.

• by Jessica Heath
• June 05, 2024

On April 8, the moon’s orbit crossed directly between the Earth and the Sun, completely eclipsing the burning star for about four minutes as the two orbs hovered over the U.S., shrouding parts of the world in eerie gray darkness for multiple hours. 

As people donned special glasses and oohed and aahed at the cosmic marvel, power grid workers and electrical engineers worked to keep the power running in the absence of solar energy. 

According to the U.S. Energy Information Administration, the eclipse partially or completely blocked sunlight to solar generation facilities with a combined 91.3 gigawatt-hours of capacity during peak solar generating time. To put it in perspective, one gigawatt-hour could power about 90,000 homes in the U.S. for a month. 

States in a northeast trajectory from Texas to Maine were affected the worst due to their position in the path of totality. During the eclipse, Texas was expected to lose between 90 and 99% of solar power generation, which makes up about 30% of the state’s electricity, and rely on other methods of electricity generation like gas, wind and coal. 

Solar panels' inability to generate electricity when the sun isn’t shining directly on them is an issue that researchers like Professor of Electrical and Computer Engineering Jeremy Munday and Professor of Materials Science and Engineering Marina Leite , both of UC Davis, are tackling in the clean energy sphere are tackling in the clean energy sphere. 

Recently, this research team has proposed a solution that would enable solar panels to produce power when the sun is not present and operate at more than 50% efficiency. Basically, the panels would be able to convert more than 50% of the energy shining on them into usable energy, a significant upgrade from the technology’s current conversion efficiency of about 20%. 

Let There Be Light and Heat

Most solar panels that can be seen on rooftops or in solar farms are made of photovoltaic cells that are made up of semiconductor materials. The cells receive photons, or light particles, from the sun and create an electric current through those materials However, there is a wide spectrum of light emitted by the sun that the panels cannot use efficiently on their own. 

The proposal: create a thermophotovoltaic device with an optical emitter to convert the sun’s heat into a concentrated light spectrum that can then be transformed into usable energy. 

“A traditional photovoltaic cell converts sunlight into electricity, but a large portion of the sun’s spectrum is not efficiently used,” Leite said. “In the case of a thermophotovoltaic device, the conversion is from heat to electricity. The thermal radiation emitted from an object called the optical emitter is absorbed by the conventional photovoltaic device.” 

The optical emitter comprises a heat source — in this instance, a rod of material like tungsten or silicon carbide — that will absorb the heat from the sun. Scott McCormack , an assistant professor of materials science and engineering, provided insight into which materials could withstand the high-heat input. The heat is converted into light, and thin films layered on top of one another shape the emission spectrum. The photovoltaic cells then absorb the light from the optical emitter more efficiently than sunlight. 

After running calculations and simulations that revealed the thermophotovoltaic device as a promising solution to solar power inefficiencies, Leite and her lab fabricated the first round of possible optical emitters in CNM2 (see page 11) for testing. Those initial emitters were then heated up to 1,500 degrees Celsius, and light shaped by thin films was emitted. 

The researchers are embarking on a second round of testing newly designed emitters with an engineered photonic filter that reflects heat back to the source and takes only the energy that is needed to create the light spectrum. They are using a carbon source to heat the emitter up to 1,350 degrees Celsius and will investigate the amount of light that reaches the photovoltaic cells and the output power from the cell using current-voltage measurements. 

“When the system is well-engineered, it will allow us to shape the spectrum in a way that minimizes the inefficient aspects of conventional solar cells and maximizes the power of the sun,” said Leite. 

Power Shift 

The filter paired with the optical emitter will also facilitate energy conversion when the sun is down. That’s because when it receives energy from the sun or other heat source, the filter reflects it back to the source and stores the rest of the energy to be used later. This could be done by covering the filter and emitter and uncovering them when energy is needed, much like when a dish of food is covered with foil to keep the heat in when it’s reheated. 

Of course, the ability to generate electricity when the sun is down could drastically increase the effectiveness of solar panels and could be a huge boon to the clean energy field. 

“In California, we have almost 10 times as much installed solar power as we did 10 years ago, which is amazing,” Munday said. “Because our technology uses a local heat source, we can help shift the power from daytime to nighttime, which could have a huge impact on further increasing the usefulness of solar energy.” 

The team is confident this system has the potential for 50% efficiency, which could fundamentally change how solar energy is harvested and stored, and steer society further toward clean energy. 

“Climate change is one of the biggest problems facing humanity today, and finding alternative power conversion technologies that can help address this problem is crucial,” said Munday. “This particular technology is an excellent new option. Together, we are super excited to see where it takes us.” 

Learn how we are revolutionizing energy systems

This article was originally featured in the Spring 2024 Engineering Progress Magazine .

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• Review Article
• Published: 23 May 2024

Monitoring, trends and impacts of light pollution

• Hector Linares Arroyo   ORCID: orcid.org/0000-0003-0034-3700 1 ,
• Angela Abascal 2 ,
• Tobias Degen 3 , 4 ,
• Martin Aubé 5 , 6 ,
• Brian R. Espey 7 ,
• Geza Gyuk 8 ,
• Franz Hölker   ORCID: orcid.org/0000-0001-5932-266X 3 , 9 ,
• Andreas Jechow   ORCID: orcid.org/0000-0002-7596-6366 3 , 10 ,
• Monika Kuffer 2 ,
• Alejandro Sánchez de Miguel 11 , 12 ,
• Alexandre Simoneau 5 , 6 ,
• Ken Walczak 8 &
• Christopher C. M. Kyba 13 , 14  

Nature Reviews Earth & Environment ( 2024 ) Cite this article

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Light pollution has increased globally, with 80% of the total population now living under light-polluted skies. In this Review, we elucidate the scope and importance of light pollution and discuss techniques to monitor it. In urban areas, light emissions from sources such as street lights lead to a zenith radiance 40 times larger than that of an unpolluted night sky. Non-urban areas account for over 50% of the total night-time light observed by satellites, with contributions from sources such as transportation networks and resource extraction. Artificial light can disturb the migratory and reproductive behaviours of animals even at the low illuminances from diffuse skyglow. Additionally, lighting (indoor and outdoor) accounts for 20% of global electricity consumption and 6% of CO 2 emissions, leading to indirect environmental impacts and a financial cost. However, existing monitoring techniques can only perform a limited number of measurements throughout the night and lack spectral and spatial resolution. Therefore, satellites with improved spectral and spatial resolution are needed to enable time series analysis of light pollution trends throughout the night.

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Towards an absolute light pollution indicator

Air pollution mitigation can reduce the brightness of the night sky in and near cities

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Acknowledgements

A.A., A.S, C.C.M.K., F.H., H.L.A., M.A., M.K. and T.D. received funding for this work through ESA’s New Earth Observation Mission Ideas (NEOMI) program under contract 4000139244/22/NL. A.S.d.M. has been funded by European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement number 847635 (UNA4CAREER). A.J. was supported by the project BELLVUE “Beleuchtungsplanung: Verfahren und Methoden für eine naturschutzfreundliche Beleuchtungsgestaltung” by the BfN with funds from the BMU (FKZ: 3521 84 1000).

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Linares Arroyo, H., Abascal, A., Degen, T. et al. Monitoring, trends and impacts of light pollution. Nat Rev Earth Environ (2024). https://doi.org/10.1038/s43017-024-00555-9

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