Studies and Reviews on Light Physiology — Why Pixun?
Studies and Reviews on Light Physiology — Why Pixun?
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Eye Health
Eye Health
Far beyond mere vision, light has pervasive physiological effects on all living tissues. The retina is both the most exposed and the most sensitive to these influences. Other structures in the eye are also strongly influenced by the spectral composition of light. Through the following list of research articles, we touch on the most important aspects of these effects from the perspective of light exposure through digital displays.
Some of these papers focus on basic mechanisms, which are often only addressable through animal experiments. We also included epidemiological studies on humans to show how these mechanisms relate to real-life exposure to various light environments. While animal models in these experiments are chosen to represent some aspects of human physiology, the extent to which they translate to everyday human health is difficult to assess in many cases. More so than any single study or mechanism, however, it is the comprehensive view of these studies that points to the importance of daylight and why efficient electric light sources fall short in terms of health.
Short-Sightedness
Short-Sightedness
A complete spectrum appears to be necessary for the development of good focus (emmetropization). In various studies, a spectrum lacking either long or short wavelengths results in near- or far-sightedness. We have a growing mechanistic understanding of how chromatic cues of blur — where different frequencies of light have different focal lengths — contribute to clear vision, with a balance between red and blue being necessary for optimal emmetropization.
Daylight exposure has positive effects on the development of good focus. Hormonal factors, eye movements, and other elements can also play a role. Nevertheless, both epidemiological and mechanistic studies show that light composition provides powerful cues for emmetropization and incomplete spectra from electric lighting and displays are likely culprits contributing to the current myopia epidemic.
Light exposure for myopia control — review
red light is the most effective form of light exposure therapy against myopia progression
Myopia and spectral considerations
incomplete light spectrum is mechanistically implicated in short-sightedness
Blue and red together lead to normal development of good focus
the strongest chromatic signals of defocus come from the short and long ends of the visible spectrum, likely guiding emmetropization
incomplete spectrum regarding both ends of the visible range can disrupt emmetropization
Low-level red light therapy to halt myopia progression
low-level red light therapy halts myopia progression over a six month period
Development of sharp vision requires broad-spectrum light
monochromatic lights play a role in emmetropization
they cause a refractive overcompensation that is much bigger than the chromatic defocus (in natural light conditions, the spectrum is always balanced - so it makes evolutionary sense that there is an open-loop regulation of eye growth control, resulting in large effects of unnatural spectra)
Chromatic cues guide the development of sharp vision and rely on both blue and red light
longitudinal chromatic aberration (different wavelengths of light being focused more forward or backward in the eye) plays an important role both in accommodation (short-term adjustments in focus) and emmetropization (long-term development of sharp vision)
narrow-band light can break these mechanisms, resulting in significant defocus
other visual cues also play some role, but chromatic cues can overwrite them
Red light against short-sightedness
depending on conditions, red light can
shift refractive status towards hyperopia
counteract forces that cause myopia
Chromatic cues have powerful effects on ocular growth!
More time outdoors — less myopia
While the exact contribution of various mechanisms is an open question, time spent outdoors helps prevent short-sightedness:
Broader spectrum — less myopia
Even broader spectrum LED helps with myopia (while, for many reasons as shown here with other studies, it cannot compete with natural light):
Serious consequences of the myopia epidemic
Daylight in schools and public health
daylight could play a role in preventing myopia
Quarantine myopia
more myopia development and worsening of preexisting myopia among children subjected to prolonged home confinement
Red and blue both needed for development
longitudinal chromatic aberration in the eye (short-wavelength light, e.g. blue, having a shorter focal length than long-wavelength light, e.g. red) gives signals for emmetropization (proper development of focus)
monochromatic light disrupts these mechanisms, resulting in refractive error
studies with different species concluded with different direction of such effects (red or blue light only resulting in hyperopia or myopia)
circadian and hormonal differences might be a confound in the comparison between species
nevertheless, across species, white light (a balance between long and short wavelengths) is needed for emmetropization and monochromatic light results in improper development
Red against myopia in monkeys
"In infant monkeys, narrow-band, long-wavelength lighting:
Produces axial hyperopia.
Prevents form-deprivation myopia.
Retards myopic compensation to imposed hyperopic defocus.
Augments hyperopia in response to imposed myopic defocus.
Chromatic cues have powerful effects on ocular growth in primates."
Reversal of myopia with red light treatment
650 nm red light was used due to its known safety and efficacy in amblyopia treatment
treatments two times daily for 3 minutes on weekdays, for a one year period, on 111 myopic children (against a control group of 114)
the treatment greatly slowed axial elongation and myopic refraction progression
for over 1/5th of participants, axial shortening was achieved, resulting in a reversal of myopia progression
Red light photobiomodulation for myopia
photobiomodulation therapy with red light (650 nm) was used on myopic children
axial length growth was significantly smaller in the treatment group
their refractive status was also better than that of the control group at 12 months
Incomplete spectrum and myopia
even broader spectrum LEDs can help with myopia (in animal model)
Spectrum and myopia — mechanisms
sharp vision is greatly driven by chromatic cues (chromatic aberration: short wavelengths are focused in front of long wavelengths, creating differences in blur on the retina)
these chromatic signals for blur are more powerful than monochromatic signals
during emmetropization (the development of good focus), such signals are used and thus the spectrum of light sources have an influence on them in various mammalian species, including humans
artificial illuminants and computer screens might reduce the effectiveness of these processes
Time outdoors and myopia
While the exact contribution of various mechanisms is an open question, time spent outdoors helps prevent short-sightedness.
We have a myopia epidemic
...with still growing prevalence and serious long-term consequences beyond just poor vision:
Blue Light and Tissue Stress
Blue Light and Tissue Stress
Short-wavelength light (violet and blue) carries more energy than other regions of the visible spectrum. It is also in the blue range that tissues are most susceptible to oxidative stress and the harmful effects of light exposure — both acutely at high intensities and at low levels when exposure is chronic and recovery is insufficient.
Overview of mechanisms of blue light harm in the retina
blue light: short wavelength, high energy, and strong penetration reaching the retina with little loss in damage potential
causes reactive oxygen species accumulation and oxidative stress, photochemical damage and negative impact on mitochondrial function
the major source of blue light is sunlight, significant exposure comes from displays and general lighting (here, the paper fails to note that due to differences in the rest of the spectrum, blue light from sunlight has a different effect than unnatural spectra from electric lighting - see relevant literature in the section below)
Deleterious effects of LED light on retinal pigment epithelium
common white LEDs emit light with high blue light content
such a light source was shown to induce damage to retinal pigment epithelial cells and various mechanisms of action were measured
structural alterations were shown, resulting in the disruption of the blood-retinal barrier
while the used animal model differs from humans, the mechanisms are similar and thus the study shows why different light sources, even when they appear similar, can have different physiological effects
Mechanisms of mitochondrial dysfunction by blue light
reactive oxygen species mediate the effects of blue light resulting in mitochondrial dysfunction and mtDNA damage
Cell defense mechanisms against blue light oxidative stress
blue light irradiation:
reduces metabolic activity
increases intracellular reactive oxygen species levels
increases expression of stress-related proteins
Mechanisms of retinal thinning and degeneration caused by blue light
blue light exposure caused a decrease in cell proliferation and an increase in apoptosis in the retinal pigment epithelium of a zebrafish model
oxidative stress increased during the early stage of 2 h of exposure and activated DNA damage in RPE cells after 8 h
autophagy was activated in response to blue light exposure at 24–48 h
blue light’s harmful effect is based on the induction of RPE cell death through a sequential mechanism of cell proliferation inhibition and the induction of DNA damage and oxidative stress
as before: this is an animal model but mechanisms apply to human cells too
Blue light's relation to age-related macular degeneration
our mechanistic understanding suggests that cumulative oxidative stress, including from blue light, is involved in the pathogenesis of AMD
animal studies confirm blue light's damaging potential
it is not clear though how this applies to macular degeneration, where epidemiological studies show mixed results
The circadian side: Exposure to artificial light at night also increases AMD risk
outdoor artificial light at night increased the risk of incident exudative age-related macular degeneration
primary mechanisms suspected behind the effect are linked to circadian disruption, including changes in hormone secretions, impaired immune functions, and proinflammatory processes
shorter wavelengths are more detrimental and the recent change towards LED sources is likely worsening this problem
Photobiomodulation is effective for AMD in the short term
short-term improvements in visual function and a reduction in drusen volume
long-term efficacy and impact on disease progression are uncertain
Mitochondrial mechanisms of photodamage in the retinal pigment epithelium
regulation of mitochondrial fission and fusion is pushed out of balance by blue light towards mitochondrial fragmentation
blue light thus decreased the viability of cells and increased reactive oxygen species production
these mechanisms have relevance to AMD pathology
Oxidative damage in plasma membrane from blue light through retinal
retinal, when excited by blue light, distorts plasma membrane bound phospholipid and disrupts its function
this in turn changes the shape of the cell and increases cytosolic calcium, resulting in cell death
blue light alone or retinal alone does not lead to the same changes
Increased screen time associated with dry eye disease
on a sample of 456, with an average weekly screen time of 44 hours, 90% symptomatic for dry eye disease
tear film stability was less than 10 s in 24% of cases
poorer symptomology correlated with increased screen use, elevated blink rates and reduced proxy tear film stability
Blue light accelerates tissue aging (in animal model)
blue light can damage cells and tissues not specialized for light perception
resulting metabolic rearrangements show impairments in energy production
lower levels of several neurotransmitters suggest disrupted brain homeostasis and earlier onset of neurodegeneration
the above mechanisms for blue light's negative effects on metabolic pathways are present in human cells similarly to the used animal model
"prolonged exposure to artificial light with a high content of blue wavelengths is a matter of increasing concern for human health, especially with regard to retinal pathologies"
Blue light restricts cell respiration and energy production (mice in vivo)
at environmental irradiance levels, blue light (420 nm) was used on mice while recording mitochondrial function and blood oxygenation in real time
significant disturbance in both mitochondrial and blood signals were recorded
the effects persisted for up to one hour after blue light exposure
longer visual recovery times in humans for short wavelengths might result from similar mechanisms
"Our developments in our artificial lighting in which we spend a large proportion of our time are increasingly biased towards short wavelengths. Given the data presented here, it seems likely that retinal metabolism would respond to these features of light in a way that we do not yet appreciate but that may impact on our vision."
Imbalance of mitochondrial dynamics in the retina by exposure to blue light (animal model)
blue LED light (450 nm) cause disruption in mitochondrial dynamics through upregulation of fission-related proteins and downregulation of fusion-related proteins
these mechanisms are involved in AMD pathogenesis
Oxidative stress, inflammation and cell death in tissues in the front of the eye by blue light
"In human corneal and conjunctival epithelial cell lines, blue light provokes cellular death and ROS production.
Specific narrow wavebands of blue light harm mitochondria and antioxidant protection system functioning.
Conjunctival cell line is more blue-sensitive than the corneal cell one.
Hyperosmolar stress impacts the phototoxicity."
Increased inflammation and cell death in the cornea by blue light in animal model
red (630 nm), green (525 nm) and blue (410 nm) LED light were compared (also to an untouched control) at the same intensity over a 10-day duration regarding its effects on the cornea and conjunctiva of mice
"Overexposure to blue light with short wavelengths can induce oxidative damage and apoptosis to the cornea, which may manifest as increased ocular surface inflammation and resultant dry eye."
in various metrics, this was true against control and red light conditions, or control and both other light conditions
Benefit of filtering blue light for dry eye patients
Display use and dry eye
dry eye is related to a large proportion of digital eye strain complaints
blue light causes oxidative damage in the cornea, contributing to dry eye disease
shorter wavelengths in the blue range have greater harm potential
these basic mechanisms are known, but their contribution in real-life cases is still an open question, where blue filtering glasses, but their testing for efficiency yielded mixed results so far
Display use safety and blue light — review
several mechanisms on blue light's effects on the eye, sensory cells, visual behavior, and circadian physiology are reviewed
despite the existence of some mechanistic understanding of various effects, it is overall unclear if filtering blue light has any protective effects
The harmful side of SAD (and other LED) lighting
electric light supplementation against seasonal affective disorder (SAD lighting) can serve its circadian entrainment and activation purpose
however, especially above 40 years of age, intense irradiation with blue (400-440 nm) light increases the risk for age-related light damage to the retina
Our comment: the cause of this issue is the unnatural spectrum of such electric light sources — more so than the mere exposure to blue light. While the circadian entrainment potential is driven by short wavelengths and thus the effects of daylight and LED (SAD) lights can be comparable, the latter has an unnaturally high tissue damaging potential due to the lack of red and near-infrared wavelength that balance out the stressor effects of blue light in any natural daylight condition.
Retina harm potential of domestic LEDs
blue-rich LED light can be harmful to the retina not only at high intensities, but also through chronic exposure at low domestic levels
in a rat model, retinal injury (apoptosis and necrosis of photoreceptors and free radical production) was found with blue (460 nm) and white LEDs, but not with fluorescent lights in a matched condition
Blue-light harm through ferroptosis
ferroptosis as one mechanism to cell death by blue light in the retina
Photodamage by blue LED light
blue light causes mitochondrial dynamics deregulation in retinal pigment epithelium cells
this has potential to contribute to age-related macular degeneration
Blue light and ocular pain
nociceptive neurons from trigeminal ganglia are responsible for eye pain
this study investigated the cytotoxic impact of various wavebands of visible light (410-630 nm) on primary cell culture of mouse trigeminal neural and glial cells (in vitro)
at the tested irradiance, only blue wavelengths provoked cell death, altered cell morphology, and induced oxidative stress and inflammation
"our results give some insight into circuit of tangled pain and photosensitivity frequently observed in patients consulting for these ocular symptoms"
Phototoxic effect of chronic white-LED light exposure (animal study)
long-term exposure to different domestic LEDs and fluorescent sources were tested on a rat model
"the blue component of the white-LED may cause retinal toxicity at occupational domestic illuminance and not only in extreme experimental conditions, as previously reported"
"current regulations and standards have been established on the basis of acute light exposure and do not take into account the effects of repeated exposure"
Display screens: shorter wavelength blue light is more harmful than longer wavelength blue light
low-intensity blue light from display devices still carry potential to increase cell death in retinal pigment epithelial cells
this effect is worse when blue light is of shorter wavelength (449 vs 458 vs 470 nm)
The "blue spike" issue in LEDs
the "blue spike" in the spectral emission of regular white (phosphor converted) LEDs contributes to excess retinal cell damage relative to blue-free (yellow+red) LEDs
The "blue spike" in LCD screens
adjusting the backlight in LCDs to reduce energy emission/short-wavelength proportion, while maintaining luminance, decreases cell damage
Interactions Between Short and Long Wavelengths
Interactions Between Short and Long Wavelengths
The puzzle: While blue light can be harmful, it is also essential to receive ample exposure to it during the daytime for proper circadian entrainment, resulting in higher activation levels while awake and better sleep at night.
The solution: Exposure to short wavelengths from sunlight always comes with an abundance of long wavelengths, which counteract the tissue-stressor aspects of blue light while still allowing for powerful circadian stimulation. These long wavelengths are completely missing in contemporary electric light sources (including displays).
Therefore, comparing the damaging potential of blue light from various sources cannot solely rely on measures of blue light. The same dose of blue light from daylight versus an LED can have similar circadian effects; however, the tissue harm potential of the latter is greater due to its unnatural spectral composition.
Similar to exercise, hunger, or other aspects of our physiology, periodic exposure to stressors is essential for healthy functioning. Avoiding short-wavelength light can lead to poor sleep at night, low energy during the day, and a long list of negative physical and psychological consequences. Instead, the damaging potential of blue light can be managed through spectral balance and timing of exposure, reflecting natural circadian dynamics of spectral changes. The studies below show why spectral balance between short and long wavelengths is important (and several studies listed under 'Systemic Health Effects' address the question of timing).
Phototoxicity of low doses of light and influence of the spectral composition on human RPE cells
blue AND white light (3.6 J/cm^2 at low doses; well below 22 J/cm^2 threshold previously thought to be toxic) induce inflammatory response and other cell mechanistic responses associated with damage
red light is shown to inhibit or down-regulate these inflammatory pathways
in combination with visible white light, red and infrared portions of the full spectrum of sunlight are critical in comprehensive modulation of inflammation and damage of RPE cells from the retina of the eye
Blue light damage underestimated, but red has protective effect
phototoxicity is not all about blue light
thresholds are overestimated in animal model
red light has protective effect, influencing overall phototoxicity
low doses of LED light can also cause significant damage to the retinal pigment epithelium
Spectral opponency reduces photoreceptor damage from short wavelengths
spectral opponency can reduce damage to photoreceptor and retinal function (in rats) with equal or greater illuminance compared to conventional white LED exposure
frequency matters: conventional LEDs have a blue peak with wavelengths prone to cause tissue stress
using other frequencies instead lowers this effect
adding long wavelengths counteracts the damage
Mitochondrial mechanisms in healing by near-infrared light therapy
inclusion of infrared parts of the spectrum could reduce the mitochondrial damage/dysfunction and related injuries and conditions
in rats, toxicity levels likely overestimated by a factor of 50 when considering blue light and by a factor of 550 concerning white light
white light toxicity may be largely driven by green wavelengths promoting an inflammatory response that initiates invasion of macrophages in the retina, potentially 8 fold higher than that of blue wavelengths
red mitigates damage, inhibits the nuclear translocation of L-DNase II and also reduces the number of TUNEL-positive cells by more than 30%
Long wavelengths counteract photochemical harm from short wavelengths
bright light was used to create photochemical damage in rat retinas
red light (670 nm) before, during, or after bright light exposure ameliorated the damage in all three conditions
"Near-infrared (NIR) photobiomodulation is protective against bright-light–induced retinal degeneration, even when NIR treatment is applied after exposure to light. This protective effect appears to involve a reduction of cell death and inflammation. Photobiomodulation has the potential to become an important treatment modality for the prevention or treatment of light-induced stress in the retina."
Long wavelengths decrease blue light damage through enhancing energy metabolism
oxidative stress was induced by blue light in an ex vivo model
the resulting mitochondria-induced apoptosis was decreased upon red or near-infrared light exposure
Near-infrared protects from UV harm
there are natural dark- and light-dependent mechanisms to protect against hazardous effects of short-wavelength radiation
broad-band infrared (700-2000 nm) radiation, in the absence of rising temperature, induces a strong cellular defense against UV cytotoxicity
Red light protects from blue light harm
retinal pigment epithelial cells in culture respond to blue light by mitochondria depolarization and increase in reactive oxygen species
when blue light irradiation is followed by red light exposure, all these effects are significantly blunted
these mechanisms have relevance to protecting the retina from blue light harm as well as in age-related macular degeneration
Oxidative stress from blue light reduced by red light
short wavelength light negatively affects mitochondrial function, causing oxidative stress and decreased cell survival in cultured retinal precursor cells
long wavelength light enhances mitochondrial function to increase survival and reduce the effects of blue light
on a long-term basis, these mechanisms might be relevant to chronic harm of the retina by blue light exposure, which can be counteracted by red light exposure
Age-related macular degeneration protection index and blue-red balance
blue light increases reactive oxygen and decreases cell survivability in the retina
these mechanisms are implicated in the formation of age-related macular degeneration (AMD)
red light can have the opposite effect and thus protect the retina from oxidative damage
to compare various light sources regarding their potential effect on AMD, a protection index is proposed to show the balance between short and long wavelengths
Our comment: nothing beats natural light (see Planckian radiators, Figure 4)
Protective Effects of Red and Near-Infrared Light
Protective Effects of Red and Near-Infrared Light
Long-wavelength light has tissue-regenerative effects through various mechanisms. This has everyday relevance for health, both through daylight exposure and clinical applications.
While the common narrative around photobiomodulation highlights beneficial properties through increased metabolism, a more detailed examination reveals that this is an oversimplification. Certain frequencies can increase cell respiration, while others may decrease it; these effects depend on a multitude of mechanisms.
The full spectrum of daylight, with all its dynamic circadian changes, provides an optimal balance of these mechanisms, while narrow-band light sources, such as LEDs or lasers, can selectively trigger certain mechanisms without also activating their antagonists. Therefore, LEDs can be used effectively for clinical purposes; however, for health preservation and maintaining natural balance, sunlight remains the most optimal choice.
Review of photobiomodulation for eye health
red and near-infrared light applied to enhance mitochondrial function, boost ATP production, reduce oxidative stress and result in improved cellular repair and tissue regeneration
in the retina, these effects can reduce cell death under stress conditions
in the cornea, these effects can help with wound healing and reduce scarring
clinical applications in age-related macular degeneration, diabetic retinopathy, dry eye disease, aiding post-surgical recovery, improving visual acuity, reducing inflammation, and improving tear film stability
General mechanisms of photobiomodulation
cytochrome c oxidase absorbs near-infrared light -> several hypotheses for how this leads to the observed clinical effects:
photons dissociate inhibitory nitric oxide from the enzyme, leading to an increase in electron transport, mitochondrial membrane potential, and adenosine triphosphate production
light-sensitive ion channels can be activated allowing calcium to enter the cell
after the initial photon absorption events, numerous signaling pathways are activated via reactive oxygen species, cyclic AMP, NO, and Ca2+, leading to activation of transcription factors
transcription factors can lead to increased expression of genes related to protein synthesis, cell migration and proliferation, anti-inflammatory signaling, anti-apoptotic proteins, and antioxidant enzymes
stem cells and progenitor cells appear to be particularly susceptible to these effects of near-infrared light
Photobiomodulation — underlying mechanism and clinical applications
red/near-infrared light can
modulate cell behaviors, enhancing the processes of tissue repair
induce cell proliferation
enhance stem cell differentiation
with uses in relieving pain, reducing inflammation, enhancing healing and tissue repair
Photobiomodulation for age-related macular degeneration — review
photobiomodulation (PBM) can improve visual function, slow the accumulation of drusen, and slow disease progression in AMD
PBM is very safe and well tolerated
Note: this review was funded by the industry; however, its conclusions are in line with several studies. In contrast, other clinicians conclude that PBM improves visual function but does not halt AMD progression. In our view, evidence from clinical studies on AMD are not conclusive regarding AMD per se, but show that mechanisms involved in the retina's ability to deal with oxidative stress are positively affected by long-wavelength light.
Red light to protect eyesight
deep red light improves declining eyesight through enhancing mitochondrial performance
Do neurons communicate through light?
photons convey important information content to cells for repair and potentially for communication
Deep red light improves declining eyesight through enhancing mitochondrial performance
the retina is particularly sensitive to oxidative stress, due to having the highest metabolic demand in the body, making it susceptible to early aging
670 nm light was used to improve mitochondrial functioning and significantly moderate this aging process
Long-wavelength light and enhanced cytokine expression
670 nm light enhanced cytokine expression in blood and retina (of mice)
cytokine expression increases with age and these findings might be part of a protective effect of long-wavelength light
however, their upregulation might be negative and we do not fully see the complex patterns of interaction, calling for further exploration
Photobiomodulation can protect the retina from oxidative stress
oxidative stress plays a role in retina aging
photobiomodulation can counteract this through its antioxidant effects
this was confirmed in a retinal pigment epithelium model with hypoxia through the use of red (660 nm) light
Photobiomodulation can reduce inflammation in age-related macular degeneration
inflammatory markers were reduced by 670 nm (red) light in a mouse model of age-related macular degeneration
this happened through an increase in cytochrome c oxidase, a mitochondrial enzyme that plays a role in oxidative phosphorylation
Photobiomodulation and sunlight — review
a long host of benefits from sunlight are attributed to vitamin D
clinical trials with vitamin D supplementation have undermined this assumption
red and near-infrared light, on the other hand, have similar benefits and a growing number of studies show such effects and their underlying mechanisms
this paper highlights that sunlight improves health through several mechanisms of light physiology, photobiomodulation being a significant and often neglected aspect of these
Photobiomodulation in general lighting
clinical and basic research show various health effects of long-wavelength light
these have long been dismissed as irrelevant for general lighting due to the high intensities used in clinical applications
thus, daylight and electric light are often compared based on their visual and circadian entrainment qualities alone
this paper summarizes the most crucial pieces of evidence to the contrary
the lack of long wavelengths in indoor electric lighting might have physiological significance
photobiomodulation effects are observable at intensity levels not dissimilar from that in natural ambient illumination
Opposing effects in adjacent frequency bands
in photobiomodulation, commonly used frequencies show an excitatory effect on cell respiration, while other frequencies are often reported as ineffective
this study shows that some frequencies in the photobiomodulation range are actually inhibitory to cell metabolism
Our comment: this line of research and its clinical applications is sadly often outside of the scope of photobiomodulation. However, it provides a crucial piece of evidence for why we have to tread carefully with narrow-band stimulation: there are a long list of mechanisms potentially at play, where even antagonistic effects can be achieved by just a slight shift in frequency. Therefore, narrow-band near-infrared supplementation for general lighting purposes is most likely not a good substitute for full-spectrum natural light and might even unforeseen consequences through triggering certain mechanisms repeatedly while their natural antagonists are absent.
Clinical application of inhibitory infrared light
certain long wavelengths have an inhibitory effect on cell respiration
this can be used to prevent reperfusion injury by inhibiting hyperactive mitochondria
Effects on Vision and Performance
Effects on Vision and Performance
Light Sensitivity
Light Sensitivity
Symptoms of digital eye strain have now affected the majority of individuals. These symptoms include dry or itchy eyes, visual fatigue or artifacts, headaches, and other forms of discomfort caused by digital displays. For some individuals, these symptoms can be debilitating.
The spectral and temporal properties of light emitted by displays appear to be significant factors contributing to these symptoms. While oculomotor aspects of reading, ergonomics, and other considerations certainly play a role, they are also present in print-based work, which does not elicit the same problems.
Effective solutions seem to be scarce. Guidelines emphasize taking eye breaks, maintaining distance from screens, and limiting exposure—practices that can be difficult for many to implement. Front-lit displays (such as e-ink or RLCD) are popular because they can be used in daylight, alleviating symptoms for many; however, their use is limited by technological constraints and low image quality. Consequently, they are primarily applicable to specific display types (e-book readers, tablets, etc.) and cannot replace regular computer screens.
Guidelines for students with light sensitivity — review
brain-related injuries like concussion, migraines, etc. and conditions like ADHD can require special accommodations in order for users (especially the young) to overcome issues preventing normal or modified usage of electronic screens
guidelines by digital media manufacturers and departments of education diverge
there is a large discrepancy between guidelines from academic
research and inclusive educational guidelines
most common recommendations:
reduce flicker
decrease luminance
avoid saturated red flashes and increase color contrast for text
use daylight where possible
take breaks from display screens and limit duration
increase distance from screens
Our comments: guidelines are constrained by both the outdated nature of their knowledge base and the technological limitations regarding reasonable adjustments that can be implemented in schools. Furthermore, practice lags behind guidelines by rarely implementing adjustments that could benefit sensitive students. Providing daylight in computer displays can address many of these issues without necessitating behavioral changes for teachers or students or requiring modifications to classroom setups.
Color overlays and physiological parameters while reading in dyslexia
dyslexic children have longer reading duration, fixation count, fixation duration average, fixation duration total, and longer saccade count, saccade duration total, and saccade duration average while reading
combinations of background and overlay colors can have positive neurological effects resulting in increased reading performance for them
turquoise background, turquoise overlay, and yellow background can be beneficial for their reading
Computer vision syndrome in professional computer users
computer vision syndrome - symptoms like irritation, blurred vision, burning or dryness, eye-strain, tired eyes, ache in the eyes lasting three or more days
prevalences can be as high as 55% among intensive computer users
mitigation: a focus on design factors such as proper adjustments of the workstation including illumination and reductions of glare and reflections; increasing viewing distance
the main conclusion is that allowing frequent breaks should be secured
Our comments: While this review is somewhat dated, it illustrates how little has changed in the past ten years, even as many studies today report a higher prevalence of symptoms—perhaps in line with the increased duration of display use. Although the oculomotor aspects of digital eye strain discussed in the paper are certainly relevant, it is noteworthy that these aspects of near work are closely matched between display and print reading, while the properties of light differ.
Vision science calls for perception-friendly displays
the majority of current color displays employ trichromatic light emission spectra to achieve realistic-looking content
enlarging the color gamut of display design can negatively impact the user’s enjoyment of natural world colors because of visual adaptation and metamerism at the expense of better color fidelity
excessive exposure to wide-gamut visual stimuli could cause adaptive responses in the human vision system that could change how people perceive the real world (that is, the vibrant colors of modern displays may appear attractive at first, but they distort perception so that the real world will look dull)
"We recognize that in side-by-side comparison, viewers often may choose a WCG [wide color gamut] display if they do not understand the associated problems. For this reason, we strongly recommend that displays should be required to emit light that is less able to cause adaptive changes in color vision, at least some of which can be relatively long-lasting."
"Effectively, these displays cause a significant portion of the color-normal population to be made, effectively, slightly color blind when viewing the display. This can be avoided by not using trichromatic displays or by deliberately broadening emission spectra."
current displays also lack deep-red and near-infrared frequencies and there is no standard metric for their contribution to the emitted spectrum
the authors suggest that these frequencies should be added to display emissions for health considerations
Computer use contributes to headaches
computer use significantly raises primary headache prevalence
Epidemic of digital eye strain in children
the trend toward online education is driving a generation towards digital eye strain
large portions of the population have symptoms like irritated or burning eyes, dry eyes, eye strain, headaches, and more
the most prevalent preventative intervention is taking pauses in between work
spectral filtering of various sorts can be somewhat helpful
Curfew increases digital eye strain
78% digital eye strain during curfew in Saudi Arabia
usage duration is a key factor (and it increased during curfew)
Our comment: the author suggests regular eye exams, decreasing screen time, using the 20-20-20 rule, and eye drops to reduce symptoms. While this is in line with current mainstream recommendations, it does not offer a solution for the actual problem, only avoidance and mitigation.
High prevalence of computer vision syndrome in virtual classrooms (Thai university students)
Out of 527 Thai students studying in a virtual classroom, 516 (97.9%) experienced at least one symptom of computer vision syndrome.
Flicker
Flicker
The rapid periodic modulation of a light source may remain imperceptible at high frequencies; however, it still negatively affects both health and visual performance.
Reading from flickering digital displays becomes slower due to disrupted planning of rapid eye movements, contributing to fatigue and often also inducing headaches or other symptoms of digital eye strain.
Please note that display flicker, in most cases, is independent of the refresh rate of the display and stems from the display's light source — not from the updating of the image.
Flicker increases arousal, alters brain activity, pupil size, and cognitive performance
Relative to a no-flicker control condition, 100 Hz and 500 Hz flicker induced changes in arousal, pupil size, EEG measures, and cognitive interference.
Flicker's effects on reading and cognitive performance, perception, and discomfort
greater modulation depth results in worse effects
regarding discomfort, the above did not to hold (in this experiment) for the relatively high frequency of 500 Hz
duty cycle affects reading errors
phantom arrays also appear for dark stimuli on bright backgrounds at high light levels - with implications to reading and many other visual tasks
How eye movements are disrupted by rapid changes in stimuli
" intrasaccadic visual information informs the establishment of object correspondence and jump-starts gaze correction"
Comment: this means that changes in the stimulus during a fast eye movement (saccade) can alter where the saccade will land, necessitating a new saccade to correct for the error. While the retinal image is blurred during a saccade, its suppression in perception is not complete (as formerly assumed), also resulting in the reading problems that we see emerge with flicker (see the other references here).
Sensitivity and flicker detection — greater annoyance and equal performance
people more sensitive to pattern glare reported greater annoyance with strobing stimuli
they were not better, however, at detecting it (in this study set-up)
Perceptibility of flicker above 1000 Hz
during fast eye movements (saccades), the flicker of a light source can be more easily perceived
in the set-up of this study, 11 participants were able to discriminate flicker at the average of ~2kHz
there is an interaction between stimulus contrast and flicker perceptibility
Perceptibility of flicker above 11000 Hz
the flickering of a small and sufficiently high-contrast stimulus can be reliably discriminated at 11 kHz
people who perform better at discriminating high-frequency flicker also report more symptoms of visual discomfort in everyday life
Systemic Health Effects
Systemic Health Effects
Light exposure has wide-ranging systemic health effects that occur through the eyes, via the bloodstream, or generated locally throughout the body.
Stressor effects (linked to any light exposure, particularly short wavelengths) and recovery effects (generally promoted by long-wavelength light and darkness at night) must be balanced over time and switch roles periodically in accordance with our natural rhythms across various timescales.
The role of daylight's dynamic changes in mood, cognitive performance, and well-being
Independent of effects on circadian phase, (simulated) gradual changes in light composition and intensity in the morning (dawn-simulating light) improved subjective well-being, mood, and cognitive performance.
Gabel, V., Maire, M., Reichert, C. F., Chellappa, S. L., Schmidt, C., Hommes, V., ... & Cajochen, C. (2013). Effects of artificial dawn and morning blue light on daytime cognitive performance, well-being, cortisol and melatonin levels. Chronobiology international, 30(8), 988-997.
The role of daylight's dynamic changes in sleep inertia
late chronotypes ("night owls") struggle with waking up on time, a phenomenon called sleep inertia
dawn-simulation helps with this
the effect is independent of the melatonin rhythm
"Mechanisms other than shift of circadian rhythms are needed to explain the positive results on sleep inertia of waking up with a dawn signal."
Near-infrared light and melatonin production
in natural light, near-infrared (NIR) impacts 60% of cells of the adult body and 100% of cells of a fetus or young child
the body has optical mechanisms to gather and localize NIR photons in the most sensitive areas
natural sunlight promotes an increase in antioxidants within our healthy cells
the cumulative effect of this antioxidant reservoir enhances the body’s capacity to quickly and locally respond to changing conditions throughout the day
subcellular melatonin, produced by mitochondria in response to near-infrared light, is a likely mediator in these effects
Mitochondrial melatonin's pervasive effects
melatonin's role in managing oxidative stress
melatonin has a major impact on mitochondrial functions through improving electron transport chain efficiency
more commonly known for its role in circadian rhythm through secretion by the pineal gland (blood melatonin; <5% of total), melatonin is also produced locally in mitochondria of all cells (mitochondrial melatonin; >95% of total)
it inhibits cancer by reprogramming glucose metabolism
Photobiomodulation as vasodilator
deep red light dilates blood vessels through increased intracellular nitric oxide bound substances from the endothelilum
this has potential treatment applications in diabetic vascular disease
Near-infrared on well-being and health in humans with sleep problems
indoor supplementation of near-infrared light (850 nm) had positive effects on mood and resting heart rate
these effects were only observed in the winter, not in the summer
Our comment: This is a unique study where the application of photobiomodulation is not for a clinical purpose and not with relatively high intensity over a short duration of time, but rather for supplementing a narrow band of the lacking (broad) band of near-infrared light in health people's regular indoor environments over longer time durations. While likely generally safe, this is an unnatural situation and we cannot assume the same effects as with full-spectrum daylight. The observed differences between winter and summer periods are interesting to compare to the thoughts of e.g. Scott Zimmerman and others, according to which photobiomodulation should have lesser effects on those who are not near-infrared deprived.
Systemic effects of local near-infrared exposure
with local photobiomodulation therapy, effects on distant locations have long been reported
mediating mechanisms might involve modulation of circulating cells or molecules, effects on the microbiome, and translation through neural signaling
we do not clearly understand these mechanisms
"remote PBM highlights the complexity of animal physiology and the importance of understanding previously unsuspected interactions between bodily tissues in the search for effective neuroprotective interventions"
Our comment: clothes can block light exposure to large parts of the body (especially in the winter). Yet, full-spectrum daylight exposure still has great systemic effects, through the eyes and the face. Providing daylight via a computer screen is thus the most efficient way of making our indoor light environment more natural.
Physiological, cognitive, and emotional effects of near-infrared in ambient lighting
2-hour lab test on 151 participants, comparing white-light only (control) to additional narrow-band deep red and near-infrared (NIR) light
NIR resulted in a positive change in parasympathetic physiology and mood and lower performance on a visual search task
the authors conclude that exclusion of NIR from general lighting could have health implications
the authors also emphasize a balance between energy efficiency and health for general lighting
Our comment: narrow-band stimulation is not the same as full-spectrum natural light exposure, but the study shows the likely negative health consequences of the lack of NIR in our indoor light environment. As demonstrated through many other publications here, narrow-band stimulation can have negative consequences. The most effective way to find balance between energy efficiency and health is through daylighting.
Restorative effects of daylight in indoor environments — review
access to daylight in indoor environments has restorative effects
these are most pronounced in affective and clinical domains
the presence of sunlight is the most promising daylight component inducing restorative effects
negative effects are rarely reported; glare/visual discomfort from direct view of the sun should be avoided
Light at night leads to obesity
exposure to artificial light at night increases obesity in children and adolescents
Light at night increases diabetes — review
exposure to both indoor and outdoor artificial light at night (ALAN) disrupts circadian rhythms, resulting in poorer sleep and an increased risk of type 2 diabetes mellitus
blue light is associated with increased insulin resistance and higher glucose levels in the evening
blocking blue light can improve insulin resistance, reduce
blood glucose levels, and enhance sleep quality
melatonin plays an important role as it reduces blood glucose levels, while its release in the evening is suppressed by ALAN
screen-time is an impactful type of indoor ALAN, as it is associated with poorer dietary choices, lower dietary quality, incrased sedentary behavior, decreased physical activity, and negative health impact
Blue light from screens — health impact
both acute and chronic screen illumination significantly disrupted sleep continuity and architecture
greater self-reported daytime sleepiness, negative emotions, and attention difficulties
screen illumination altered circadian rhythms, subduing the normal nocturnal decline in body temperature and dampening nocturnal melatonin secretion
even one night of screen light exposure is sufficient to cause negative effects on health and next-day performance
Device-specific screen time and poor health behaviors
more screen-time, poorer dietary habits and health behaviors
heavy usage of TV and smartphone appears to be the worst
ANSES’s recommendations for limiting exposure to blue light
the French agency for all things safety:
recommends limiting blue-rich LED use, particularly via screens and especially for children
confirms the toxicity of blue light on the retina
highlights that evening exposure to blue light disrupts sleep and biological rhythm
https://www.anses.fr/en/content/leds-anses’s-recommendations-limiting-exposure-blue-light
Blue light increases, red light decreases circulating blood glucose levels (animal model)
long wavelengths increase metabolism (and higher mitochondrial respiration results in a greater demand for glucose)
short wavelengths suppress metabolism (and thus decrease demand for glucose)
as a consequence, blue light (420 nm) elevates systemic glucose levels (by over 50% in the present animal experiment), while red light (670 nm) reduces it
Red light reduces blood glucose following sugar consumption (human study)
red light (670 nm) was shone on the back of humans for 15 minutes
this decreased maximum glucose levels reached after glucose consumption
Photobiomodulation to improve insulin signaling
red (665.25 nm) and near-infrared (850.3 nm) light improves intracellular insulin signaling in animal model
this can have applications in insulin resistance treatment
Light at night suppresses day-night heart variability — review
artificial light at night (ALAN):
reduces the day-night variability of blood pressure and heart rate
is a stress-inducing factor
induces molecular changes in the heart and blood vessels
Blue light in the evening from display causes melatonin suppression
blue-light exposure before sleep:
did not lead to any change in subjective sleepiness
significantly reduced melatonin secretion
adolescents recovered more quickly from melatonin attenuation than adults
avoiding smartphone use in the last hour before bedtime is advisable
90% of studies find association between screen time and worse sleep in the young — review
screen time is adversely associated with sleep outcomes
negative effect is mainly shortened sleep duration and delayed timing
limited data on how screen characteristics affect these results
in 2011, 77% of American adolescents polled reported having difficulty waking up, feeling un-refreshed (59%), and difficulty falling asleep (42%) compared to 45% in 2006
Screen time and sleep in Chinese preschool children
J-shaped association between TV viewing time and the risk of sleep disorder, with a threshold of 1 h/day
for each 1 h/day increment in TV viewing time over the threshold, the risk of sleep disorder increased by 12.35%
Screen time and sleep in Finnish preschool children
more screen-time: later bedtimes and shorter sleep
an hourly increase in total screen time was associated with 11 min later bedtime and 10 min shorter sleep duration
this association was found for each screen type
Screen time and sleep in American preschool children
"children who engaged in more screen time were significantly more likely to have more trouble falling or staying asleep, be tired during the day, and had worse quality of sleep"
Screen time is, sitting is not, associated with worse sleep
"compared to participants with the least screen time (<2h/ day), participants with the most screen time (>6h/day) were more likely to report trouble falling asleep"
Evening screen use and sleep
Light from a display in the last hour of the day, relative to reading a book in dim light, resulted in:
reduced melatonin secretion and evening sleepiness
later timing of the circadian clock
reduced alertness the next morning
Our comment: while this study did not have detailed controls for light properties in the compared reading conditions, it still shows that display use in the evening (and the various light properties of one's reading habits) can negatively affect sleep.
Blue content in screen light affects sleep
while keeping display luminance constant, low and high melanopic settings were tested (less or more blue light in the range that most strongly affects circadian entrainment)
the low melanopic condition resulted in shorter time to fall asleep, less melatonin suppression in the evening, and lower alertness
Circadian effects and spatial properties of the light source
circadian effects of blue light are most pronounced at the center of the visual field (projected to the macula of the retina)
therefore, computer displays are the most efficient lighting application to achieve such effects (for good or for bad)
Software solutions are insufficient against sleep disruption from display use
changing the spectral composition of displays (e.g. through "Night Shift" or other applications) are not enough to reduce melatonin suppression from evening display use
such changes need to be combined with marked dimming of light intensity of the display
Increasing screen time is a risk factor for obesity in the young — review
Screen time over 2 hours/day carries significantly higher risk for obesity than less than 2 hours/day in children under 18.
Light at night is increases risk and burden of metabolic disease
cumulative association between outdoor artificial light at night and metabolic disease show that a quarter of metabolic disease cases can be attributed to light at night
men between 46 and 59 are especially affected
Negative effects of display use on youth
light from displays can be harmful especially to the young
the French Académie Nationale de Médecine recommends
filtering of blue light
restrict or ban the use of screens at night
educate pupils and parents about the risks of screen use
Display screens and sleep — review
90% of studies find association between screen time and worse sleep
Display use and sleep
digital display use in the evening leads to insomnia and later sleep
Digital display use and sleep problems
a large portion of adolescents experience permanent social jetlag
display use is an important contributor
this is a matter of public health
adolescents and their parents should be educated on sleep hygiene including avoiding displays and bright light exposure before sleep
Skin Effects
Skin Effects
The effects on skin related to displays are often overlooked. Nevertheless, the face is exposed to the light from computer screens just as the eyes are.
Given that the deep physiological effects of light are similar across various types of tissue, studies on the effects of light on skin yield findings comparable to those focused on the eyes. However, the literature primarily relates to our mechanistic understanding, and estimating the magnitude of these effects in real-life conditions is challenging due to a lack of studies.
Effects of blue light on human skin tissue in vitro
daily exposure to blue light produces dose-and-time-dependent changes in biomarkers associated with skin damage
blue light is associated with inflammation and oxidative stress
chronic exposure decreases the expression of genes responsible for maintaining skin barrier and tissue integrity and increases biomarkers of ageing, inflammation, and tissue damage
Blue light and ultraviolet — similar effects
blue light induces oxidative stress in the skin, preferentially in mitochondria
this is similar to the effects of UV light, while dissimilar from other parts of the visible spectrum
in human keratinocyte mitochondria, blue light induces 25% of the oxidative stress effect of UV
Our comment: the authors also suggest to protect the skin from this damage by sunscreens that not only block UV but also blue light. As shown through many other studies on this page, it is important to note that the abundance of long wavelengths in sunlight counterbalances the oxidative stress from short wavelengths, while this cannot be said about contemporary electric light sources.
Blue light and mechanisms of skin DNA damage
exposure to blue light decreases cell viability
apoptotic features include depolarized mitochondrial membranes and enhanced caspase-3 activity (inhibiting the latter can rescue blue light-induced cell death)
blue light can trigger apoptosis and some of its effects are similar to those of ultraviolet light
Skin stress, aging and wrinkles from devices?
"exposure of human skin cells to light emitted from electronic devices, even for exposures as short as 1 hour, may cause reactive oxygen species (ROS) generation, apoptosis, and necrosis"
Modulation of skin microbiome by blue light
skin microbiome responds to blue light through expression of chromophores
these modulate physiological responses, from cytotoxicity to proliferation
this can have anti-bacterial uses, but "human skin and hair follicle microbiota is a universe of many different species, one needs to restore the ecosystem balance instead of eradicating all life"
there are numerous blue light sensing systems in the skin microbiome and we do not understand much of how these work
Sun exposure enriches skin microbiome reducing oxidative damage
seasonal sun exposure can enrich specific skin microbiome bacteria
these have the ability to protect against UV radiation through reducing reactive oxygen species levels in keratinocytes
Photobiomodulation and skin — review
various frequencies in different application areas
many mechanisms involved and we don't yet see through the complexity of cellular mechanisms
aside from effector cells, skin microenvironment is also crucial
Long-wavelength light for skin rejuvenation
red (660 nm) and amber (590 nm) light can improve collagen synthesis
both frequencies reduced face wrinkle volume by ~30% without affecting skin hydration or viscoelasticity
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