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Longevity Library · Lab reference

Written by Daniel Tagge, MD
Updated October 23, 2026
Reviewed October 24, 2026
48 sources · Measured in hours of outdoor daylight a day

System: Chronobiology

Morning light and circadian regularityLight exposure · Daylight · Light at night · Circadian rhythm

In 88,905 adults who wore a wrist light sensor for a week and were followed about 8 years, those in the brightest tenth of nights had 21 to 34 percent higher all-cause mortality and those in the brightest tenth of days 17 to 34 percent lower, compared with the darker half; among about 400,000 adults, each additional hour spent outdoors by day went with lower odds of depression, tiredness and insomnia symptoms and an earlier body clock. An 18-author expert consensus recommends at least 250 melanopic lux at the eye through the day indoors, no more than 10 in the 3 hours before bed and no more than 1 while asleep. In experiments, one night of 100 lux during sleep raised next-morning insulin resistance, and a week without electric light moved the body clock about 2 hours earlier. No trial has tested light against a hard outcome.

Light is the signal that sets the body clock, and the clock times cortisol, melatonin, body temperature and how the body handles glucose across the day. Indoor life has dimmed the days and brightened the nights; the cohorts find brighter nights and darker days with higher mortality, more depression, more diabetes and more weight gain, and the trials show that a bright morning advances the clock while a lit evening or a lit bedroom delays it and moves next-morning insulin. A fixed wake time, daylight soon after it, a dim evening and a dark room are the levers; the consensus numbers are the only targets this page carries.

01 · The scale

Where the laboratory range and the evidence sit.

Brighter nights and darker days predicted higher mortality in 88,905 adults wearing light sensors; each extra hour outdoors went with better mood and sleep in about 400,000; bedroom light of 5 lux or more went with more depression and diabetes in older adults; one night of 100 lux in sleep raised insulin resistance by about 15 percent in a trial; bright light treats depression in randomized trials. No trial has tested light against a hard outcome.

0123456

  1. Typical indoor-living adult: about an hour a day at or above 1,000 lux (median 58 minutes)

    0 to 1 hours of outdoor daylight a day · Espiritu 1994

  2. UK Biobank self-report: median 2.5 hours a day outdoors (interquartile range 1.5 to 3.5)

    1 to 3 hours of outdoor daylight a day · Burns 2021

  3. Each additional hour outdoors by day: lower odds of depression, tiredness and insomnia symptoms, earlier clock; no ceiling studied

    3 and above hours of outdoor daylight a day · Burns 2021

  • Evidence supports
  • Guideline goal or unstudied
  • Higher risk in studies
Scale in hours of outdoor daylight a day. Hours outdoors by day is the quantity people can count and the one the largest cohort asked about. The expert consensus writes its targets in melanopic lux at the eye, which almost nobody measures outside a laboratory; those figures are quoted where they appear, and the wrist sensors in the mortality cohort recorded ordinary lux, not melanopic lux, at the wrist rather than the eye. Bands show where studies found something, not a recommendation for any one person.

02 · What it is

What morning light and circadian regularity does.

The eye carries a third class of photoreceptor besides rods and cones. In 2002 two groups showed in rodents that a subset of retinal ganglion cells containing the pigment melanopsin responds to light on its own, with rod and cone input blocked, and sends its axons to the suprachiasmatic nucleus of the hypothalamus, the body's master clock, and to the nuclei that run the pupil reflex; the sensitivity, spectral tuning and slow kinetics of these cells matched the known properties of circadian entrainment. In humans the pathway survives without sight: among 11 blind patients with no conscious perception of light, bright light still lowered plasma melatonin by an average of 69 percent in 3, against 66 percent in sighted controls, and those 3 slept normally and kept a 24-hour rhythm, while the other 8, 7 with no suppression and 1 with undetectable melatonin, reported a history of insomnia and 4 of them had a temperature rhythm no longer tied to the 24-hour day. Light reaching this system, not light seen, is what this entry is about.1,2,3Grade C

The clock needs resetting every day because it does not run at 24 hours. Under carefully controlled lighting the intrinsic period of the human pacemaker, measured from melatonin, core body temperature and cortisol, averaged 24.18 hours in young and older adults alike, a tight distribution in line with other species. Light does the resetting, and its effect depends on the hour: in 21 healthy adults studied under constant conditions, a 6.7-hour exposure to about 10,000 lux delayed the clock when it fell before the core body temperature minimum, in the evening and early night, advanced it when it fell after, in the morning, and produced a phase response curve with a peak-to-trough span of 5.02 hours and no dead zone during the day. Morning light pulls the clock earlier; evening light pushes it later.4,5Grade C

The standard unit of light, the photopic lux, is built on the cone system that peaks near 555 nm, and it misreads the clock's input: 6.5 hours of monochromatic light at 460 nm produced twice the circadian phase delay and twice the melatonin suppression of 555 nm light at equal photon density, which is why the authors call photopic lux inappropriate for quantifying the drive that resets the pacemaker. An international standard now weights light by the melanopsin spectrum as melanopic equivalent daylight illuminance, and the 2022 expert consensus that writes its recommendations in that unit is quoted in the range section below. The practical translation is that daylight, which is rich in short wavelengths and far brighter than any room, is the strongest everyday stimulus the clock receives.6,7Grade A

03 · Origins

Where it came from, and where it sits in the network.

Melatonin suppression by light was first shown in humans in 1980, in six healthy subjects: bright artificial light suppressed nighttime melatonin while ordinary room light did not, which established that the human response to light is qualitatively like that of other mammals. The photopigment behind it was mapped two decades later. In 72 healthy adults tested 627 times with monochromatic light between 420 and 600 nm at 2 to 3:30 in the morning, melatonin suppression fit an opsin template with its most potent region at 446 to 477 nm, distinct from the rod and cone pigments; a second group, in 22 volunteers and 215 trials, found the best fit to a rhodopsin template peaking at 459 nm and the first direct evidence of a non-rod, non-cone photoreceptor in humans. The same year the melanopsin ganglion cells were characterized in rodents and shown to project to the suprachiasmatic nucleus. The human eye, in other words, was built to read the blue-rich sky of morning and the darkness of night, and to tell the brain which it is.8,9,10,2Grade C

What electric light did to that signal has been measured directly. Eight adults who spent a week in their usual routines in Colorado in July, then a week camping in tents with only sunlight and campfires, averaged 979 lux across waking hours in the built environment and 4,487 lux while camping, more than fourfold, with 3,074 against 934 lux in the first 2 hours after waking; the only time of day they saw more light indoors than in camp was between sunset and sleep. After the camping week every marker of internal time was about 2 hours earlier, melatonin onset fell near sunset and melatonin offset before waking just after sunrise, and the later chronotypes advanced the most, which made the group more alike. A follow-up found the same in winter, with the biological night expanding in the natural 9 hour 20 minute day, and showed that a single weekend of camping achieved about 69 percent of the week's shift. The ordinary case is dimmer still: among 106 San Diego adults aged 40 to 64 wearing a light recorder, the median person spent 4 percent of the time, about 58 minutes a day, at or above 1,000 lux. Before electric light humans lived mostly in light above 300 lux or below 30; today they spend hours a day, above all in the evening, in the 30 to 300 lux band where the clock is most variably sensitive.11,12,13,14Grade C

On the shelf, light sits upstream of several markers at once. Melatonin is the most direct: in 116 young adults, ordinary room light of under 200 lux in the 8 hours before bed, against dim light of under 3 lux, delayed melatonin onset in 99 percent of them and shortened melatonin duration by about 90 minutes. Cortisol answers to the same signal in the morning: in 14 men, 800 lux for the first hour after waking raised the awakening cortisol peak about 35 percent above darkness, with no effect in the evening, and in 8 men a 3-hour bright exposure from 5 to 8 in the morning produced an immediate rise of more than 50 percent in cortisol that an afternoon exposure did not. Short-wavelength light in the late evening raised core body temperature and heart rate along with alertness in a wavelength-dependent way. Glucose handling is timed by the same clock: in healthy adults, the glucose rise after an identical meal was 17 percent higher at 8 in the evening than at 8 in the morning, independent of sleep and fasting, with early-phase insulin 27 percent lower. And mood has its own wiring: in mice, light's effect on learning ran through melanopsin cells projecting to the suprachiasmatic nucleus, while its effect on mood required a separate pathway to a thalamic region named the perihabenular nucleus, shown under a 3.5-hour light-dark cycle designed to disturb the animals without changing total sleep. Cortisol, fasting glucose, fasting insulin and HOMA-IR each have an entry here; light is a lever on every one of them.15,16,17,18,19,20Grade C

04 · The usual guidance

What the guidelines say.

Expert consensus (Brown 2022): at least 250 melanopic lux at the eye through the day indoors, no more than 10 melanopic lux from 3 hours before bed, and no more than 1 melanopic lux during sleep; no guideline sets a number of hours of daylight.

There is no laboratory range for light; the nearest thing is an expert consensus. Eighteen researchers in lighting, photometry, sleep and circadian science met at a 2019 workshop, worked through structured questions on the physiology, and published recommendations in 2022 in melanopic equivalent daylight illuminance measured at the eye in the vertical plane at about 1.2 m, seated eye height. Through the daytime the recommended minimum is 250 melanopic lux, with daylight used first and electric light enriched in short wavelengths if it is needed. In the evening, starting at least 3 hours before bedtime, the recommended maximum is 10 melanopic lux, with light depleted in short wavelengths. The sleep environment should be as dark as possible, with a recommended maximum of 1 melanopic lux, rising to 10 only when an activity at night needs vision. The panel adds that a stable and regular daily light-dark cycle is itself likely to reinforce alignment of the clock.7Grade A

The consensus figures are indoor-lighting targets drawn from laboratory physiology, not thresholds from outcome trials, and the panel says so; it also notes that the aging lens cuts the retinal dose of a given melanopic lux by about half relative to the young standard observer, so an older eye needs more. For scale: the panel cites field measurements putting ordinary evening illuminance indoors at about 30 photopic lux, which many warm white lamps already bring under the 10 melanopic lux line, and typical daytime indoor conditions in the trials it reviews below 150 melanopic lux, short of the 250 it asks for. The camping study's participants averaged 4,487 lux across the waking day under the sky, which is why the panel asks for daylight first.7,11Grade C

Hours outdoors has no guideline either. The one large cohort that asked found UK Biobank participants reporting a median of 2.5 daylight hours outdoors a day, with an interquartile range of 1.5 to 3.5, and graded its outcomes per additional hour rather than naming a threshold. Regularity of the schedule is the other half of this entry, and it does have a measure: the Sleep Regularity Index, scored from a wrist recorder, which in 60,977 UK Biobank adults predicted mortality more strongly than how long people slept. The sleep entry carries that evidence in full; here it is the reason a fixed wake time is treated as a light lever, since the wake time decides when the morning light arrives.21,22Grade B

05 · The evidence

What the studies support.

The largest study measured light on the person. In the UK Biobank, 88,905 adults with a mean age of 62 (57 percent women) wore a light sensor on the dominant wrist for 7 days between 2013 and 2016, about 13 million hours of recording in all, and were followed a mean of 8.0 years, during which 3,750 died. Factor analysis split each person's 24-hour profile into day light (7:30 in the morning to 8:30 at night) and night light (12:30 to 6 in the morning). Compared with the darker half of nights, people in the 70th to 90th percentiles of night light had 15 to 17 percent higher all-cause mortality and those in the brightest tenth 21 to 34 percent higher; compared with the darker half of days, people in the 50th to 70th, 70th to 90th and brightest tenth of day light had 10 to 16, 16 to 26 and 17 to 34 percent lower all-cause mortality. The ranges span three levels of adjustment, up to age, sex, ethnicity, photoperiod, socioeconomic position, physical activity, social activity, smoking, alcohol, urbanicity, shift work and baseline cardiometabolic health. Cardiometabolic deaths carried the larger hazards: 1.33 to 1.46 for the brightest tenth of nights and 0.61 to 0.76 for the brightest tenth of days. A model of the circadian pacemaker driven by each person's light record found lower amplitude and both early and late phase associated with higher mortality.23Grade B

Mood and sleep track daytime light across half a million people. UK Biobank participants reported a median of 2.5 daylight hours outdoors a day; each additional hour was associated with lower odds of lifetime major depression (odds ratio confidence interval 0.92 to 0.98) and antidepressant use, less anhedonia and low mood, greater happiness (1.41 to 1.48), lower neuroticism, greater ease of getting up (1.46 to 1.49), less frequent tiredness (0.80 to 0.82), fewer insomnia symptoms (0.94 to 0.97) and an earlier chronotype (0.75 to 0.77), independent of demographic, lifestyle and employment factors, with the longitudinal models pointing the same way at smaller effect sizes. In the 86,772 of those participants with sensor data, greater night light was associated with higher risk of major depressive disorder, generalized anxiety disorder, post-traumatic stress disorder, psychosis, bipolar disorder and self-harm, and, independent of night light, greater daytime light with lower risk of major depression, post-traumatic stress disorder, psychosis and self-harm, robust to adjustment for photoperiod, physical activity, sleep quality and cardiometabolic health. Both are observational; the second is cross-sectional.21,24Grade B

Light in the bedroom has been measured in older adults and followed forward. In Nara, Japan, 863 adults with a mean age of 71.5 and no depressive symptoms at baseline had bedroom light recorded; over a median of 24 months, 73 developed depressive symptoms, and those whose bedroom averaged 5 lux or more at night (153 people) had a hazard ratio of 1.89 against those below 5 lux, 1.72 after adjustment for hypertension, diabetes and sleep, with the same pattern at a 10 lux cutoff. In the same cohort, 678 adults without diabetes were followed a median of 42 months; 19 developed diabetes, and the 128 with 5 lux or more had an incidence rate ratio of 3.74 (95 percent confidence interval 1.55 to 9.05), still 2.74 when the line was dropped to 3 lux. Nineteen cases is a small number and the interval is wide. In the United States, 43,722 women aged 35 to 74 who were not shift workers reported the light they slept with; over a mean of 5.7 years, any light during sleep carried a relative risk of 1.19 for incident obesity, and sleeping with a television or a light on in the room carried 1.17 for gaining 5 kg or more, 1.22 for incident overweight and 1.33 for incident obesity against no light, after adjustment including sleep duration and quality, diet and physical activity.25,26,27Grade B

Timing of light and body weight correlate in smaller and cruder studies. In 54 adults wearing a wrist light recorder for 7 days, the mean clock time of light above 500 lux correlated with body mass index (r = 0.51) and remained an independent predictor after adjustment for sleep timing and duration, explaining about a fifth of the variance: the later in the day a person's bright light came, the higher the index. Across 98,658 Chinese adults from 162 sites, outdoor light at night estimated from satellite data was associated with higher HbA1c, fasting and 2-hour glucose and HOMA-IR and lower beta-cell function, and the brightest fifth of sites carried a diabetes prevalence ratio of 1.28 against the darkest, 1.07 per quintile, after adjustment including body mass index. Satellite radiance over a study site is a poor stand-in for light at anyone's eye, and the design is cross-sectional.28,29Grade C

The experiments are short and point the same way as the cohorts. Twenty healthy young adults slept two nights in a laboratory; the ten randomized to 100 lux of overhead room light on the second night, against under 3 lux, had a change in HOMA-IR that differed from the dim-light group (about 15 percent higher against about 4 percent lower), a Matsuda insulin sensitivity index about 16 percent lower against 3 percent higher, higher heart rate and lower heart rate variability through the night, more light sleep and less slow-wave and REM sleep, and melatonin levels that did not differ between conditions; the higher sympathetic tone during sleep tracked the higher insulin the next morning. In 19 healthy adults living 4 days in dim light, 3 hours of blue-enriched light starting half an hour after waking or 10.5 hours after waking both raised total insulin area and HOMA-IR against the dim baseline and reduced sleepiness, and the evening exposure also raised the glucose peak; the authors call the mechanism unclear.30,31Grade C

When the whole schedule moves against the clock the metabolic cost is larger. Ten adults who ate and slept on a 28-hour day for 10 days, so that behavior cycled through every phase of their internal clock, showed, when about 12 hours out of phase, leptin 17 percent lower, glucose 6 percent higher despite insulin 22 percent higher, a completely reversed daily cortisol rhythm, mean arterial pressure 3 percent higher and sleep efficiency 20 percent lower; 3 of the 8 with complete data had post-meal glucose in the prediabetic range. A later protocol separated the two effects: the circadian system alone made post-meal glucose 17 percent higher in the biological evening than the biological morning through 27 percent lower early-phase insulin, and misalignment itself added 6 percent through lower insulin sensitivity. This is the laboratory version of shift work and of a late, bright evening followed by an early alarm.32,19Grade B

The evening dose that matters is far lower than people assume. In 23 healthy young adults, a single 6.5-hour exposure in the early biological night shifted the clock and suppressed melatonin along a logistic dose-response curve, and half of the maximal delay produced by about 9,000 lux was obtained with just over 1 percent of it, dim room light of about 100 lux. In 55 adults aged 18 to 30 given 5 hours of evening light at 10 to 2,000 lux on separate nights, the group dose for 50 percent melatonin suppression was 24.6 lux, and 10, 30 and 50 lux delayed the apparent melatonin onset by 22, 77 and 109 minutes; the individual dose ranged from 6 lux in the most sensitive person to 350 in the least, more than fiftyfold. Room light of under 200 lux in the 8 hours before bed delayed melatonin onset in 99 percent of 116 young adults and shortened its duration by about 90 minutes, and light during the usual sleep hours suppressed melatonin by more than half in 85 percent of trials. These are melatonin and clock endpoints in young volunteers over nights, not disease endpoints over years.33,14,15Grade C

Light as treatment has trial evidence for depression. A meta-analysis of randomized trials through 2003 found bright light reduced depression severity in seasonal affective disorder across eight studies (effect size 0.84, 95 percent confidence interval 0.60 to 1.08), dawn simulation across five (0.73), and bright light in nonseasonal depression across three (0.53, 0.18 to 0.89), while bright light added to antidepressant medication in nonseasonal depression was not effective across five (minus 0.01); the authors note that only 13 percent of published studies met their quality criteria. A later 8-week randomized trial in 122 adults with nonseasonal major depression of at least moderate severity compared a 10,000 lux white light box for 30 minutes each early morning, fluoxetine 20 mg, both, and a double placebo: depression scores fell by 13.4 points with light, 8.8 with fluoxetine, 16.9 with the combination and 6.5 with placebo, light alone (d = 0.80) and the combination (1.11) beat placebo and fluoxetine alone (0.24) did not, and remission was reached by 43.8 percent with light, 58.6 with the combination, 19.4 with fluoxetine and 30.0 with placebo, with all treatments well tolerated. In 46 adults with bipolar depression on stable antimanic medication, 6 weeks of 7,000 lux midday light produced remission in 68.2 percent against 22.2 percent with dim red placebo light, with no switches into mania or hypomania in either group.34,35,36Grade B

Regularity has its own evidence, and it travels with light. In 60,977 UK Biobank adults wearing a wrist recorder, higher sleep regularity was associated with 20 to 48 percent lower all-cause mortality across the four more regular fifths against the least regular, and it predicted mortality more strongly than duration. Social jetlag, the gap between the midpoint of sleep on work days and free days, was named in a 2006 survey of 501 volunteers that found late chronotypes carrying the largest work-to-free-day differences and the strongest correlations with poor sleep quality, low wellbeing and smoking, most of it attributable to the misalignment rather than to chronotype itself. A large epidemiological survey found social jetlag associated with higher body mass index beyond the effect of sleep duration, and in 815 non-shift-working adults of the Dunedin cohort, social jetlag was associated with clinically assessed measures of metabolic dysfunction and obesity, and, among the metabolically unhealthy obese, with higher HbA1c and an inflammatory marker. Smartphone data from users around the world found that people who reported spending time in outdoor light went to sleep earlier and slept more than those reporting indoor light, that social pressure delays bedtime rather than wake time, and that age is the main determinant of sleep timing.22,37,38,39,40Grade B

Shift work is the strongest and best-studied misalignment. In the UK Biobank, current night shift workers had higher odds of type 2 diabetes than day workers after adjustment, 1.15 for those working none or rare nights, 1.18 for some nights and 1.44 for usual nights, with permanent night workers at 1.09 and not significant; over a working lifetime, more than 8 night shifts a month carried odds of 1.36 against never, with a significant trend by number of nights, and genetic risk for diabetes was not modified by shift work. In 2019 an IARC working group of 27 experts from 16 countries classified night shift work as probably carcinogenic to humans (Group 2A), on sufficient evidence in experimental animals, strong mechanistic evidence and limited evidence from human studies.41,42,43Grade B

06 · What moves it

The levers with evidence behind them.

What is listed here has been tested. The size of each effect, and the size of the study, are in the sentence, so a small effect reads as a small effect.

  • Outdoor light in the first hours after waking

    advances the clock, raises the morning cortisol peak, and anchors sleep timing

    Light after the core body temperature minimum, which for most people means the first hours after waking, advances the clock along the phase response curve. The camping study's participants saw 3,074 lux in the first 2 hours awake under the sky against 934 indoors, and their melatonin onset came about 2 hours earlier after a week of it. In 14 men, 800 lux for the hour after waking raised the awakening cortisol peak about 35 percent above darkness with no effect in the evening, and in 8 men a bright exposure from 5 to 8 in the morning raised cortisol more than 50 percent while an afternoon exposure did nothing. Among 109 federal office workers wearing a circadian light meter for 7 days in winter and summer, high circadian-effective light in the morning went with shorter time to fall asleep, especially in winter, better sleep quality and stronger entrainment. Outside, early, is the cheapest instruction on this shelf.5,11,16,17,44Grade C

  • A window seat, and a walk at lunch

    more daytime light, longer sleep in office studies

    Daylight indoors is weak but not nothing. Among 49 office workers, the 27 in windowless workplaces scored worse than the 22 with windows on vitality and role limitation due to physical problems and on overall sleep quality; in the 21 wearing a recorder, the windowed group had more light exposure through the workweek, a trend to more physical activity and longer sleep. In the federal buildings study, high circadian-effective light through the entire day went with better entrainment, less depression and better sleep quality. In UK Biobank, each additional hour outdoors was associated with greater ease of getting up and less frequent tiredness. The consensus asks for daylight first and 250 melanopic lux at the eye through the day, a level ordinary office lighting, below 150 melanopic lux in the studies the panel reviews, rarely reaches.45,44,21,7Grade C

  • Dim evenings, from about 3 hours before bed

    protects melatonin onset and keeps the clock from drifting later

    The consensus sets a maximum of 10 melanopic lux at the eye from at least 3 hours before bedtime, with light depleted in short wavelengths, and notes that ordinary evening illuminance indoors of about 30 photopic lux under warm white lamps often already meets it. The dose-response work shows why the hour matters more than the fixture: about 100 lux of room light delivered half the clock-delaying effect of 9,000 lux, 10 lux delayed melatonin onset by 22 minutes and 50 lux by 109 in the average young adult, and 8 hours of room light before bed shortened the night's melatonin by about 90 minutes. Lamps low and warm, overheads off, is the practical form.7,33,14,15Grade B

  • Screens at night

    delay sleep and the clock; the dose is brightness, duration and hour, not the device

    Reading on a light-emitting device in the hours before bed, against a printed book in the same people, lengthened the time to fall asleep, reduced evening sleepiness, suppressed melatonin, delayed the circadian clock and lowered next-morning alertness; a survey of 1,508 American adults found 90 percent using some electronic device within an hour of bedtime at least a few nights a week. Monochromatic light at 460 nm in the late evening suppressed melatonin and raised alertness, core body temperature and heart rate more than 550 nm light at equal photon density, which is the wavelength argument against bright, cool screens late. The individual dose for half-maximal melatonin suppression ranged from 6 to 350 lux, so the same screen in the same room is a different stimulus to different people. The trials here varied brightness and wavelength; none tested a screen's color-shifting setting against a hard outcome.46,18,14Grade C

  • A dark bedroom

    the consensus maximum is 1 melanopic lux; bedroom light of 5 lux or more tracked depression and diabetes, and 100 lux moved next-morning insulin

    The consensus asks for a sleep environment as dark as possible, with a maximum of 1 melanopic lux at the eye and 10 only when vision is needed at night. Bedrooms averaging 5 lux or more at night carried a hazard ratio of 1.72 for new depressive symptoms and an incidence rate ratio of 3.74 for new diabetes in older Japanese adults, with the diabetes estimate resting on 19 cases; women who slept with a television or light on in the room had a relative risk of 1.33 for incident obesity over 5.7 years; and one night of 100 lux during sleep raised HOMA-IR about 15 percent and lowered the Matsuda index about 16 percent in young adults, with melatonin unchanged and sympathetic tone raised. Blackout blinds and a dim, warm, low path light for the night are the whole intervention.7,25,26,27,30Grade B

  • A fixed wake time, seven days a week

    sets when the morning light arrives; regularity predicted mortality better than duration

    The wake time decides when the clock meets daylight, and holding it steady is the lever that costs least. In 60,977 adults with wrist recorders, the more regular four-fifths had 20 to 48 percent lower all-cause mortality than the least regular fifth, and regularity outperformed duration. Social jetlag, the weekend shift of the sleep midpoint, was associated with higher body mass index beyond sleep duration in a large survey, and with clinically measured metabolic dysfunction in 815 non-shift-working adults in Dunedin. Smartphone users who reported time in outdoor light went to bed earlier and slept longer than those reporting indoor light, and bedtime, not wake time, was the lever society moves; a fixed wake time with light after it is the counterweight. The consensus panel adds that a stable, regular light-dark cycle is itself likely to reinforce alignment.22,38,39,40,7Grade B

  • A light box in the dark months

    treats seasonal and nonseasonal depression in randomized trials; a physician's tool, not a substitute for the sky

    Bright light reduced depression severity in seasonal affective disorder across eight randomized trials (effect size 0.84) and in nonseasonal depression across three (0.53); in the 8-week trial of 122 adults with nonseasonal major depression, a 10,000 lux white light box for 30 minutes each early morning outperformed placebo alone (d = 0.80) and with fluoxetine (1.11), with remission in 43.8 percent on light alone against 30.0 on placebo; midday 7,000 lux light for 6 weeks produced remission in 68.2 percent of adults with bipolar depression on antimanic medication against 22.2 percent on dim red light, with no mood switches. The trials ran 6 to 8 weeks. The safety review that accompanies the method asks clinicians to stay vigilant for emergent hypomania and autonomic over-activation, especially in the first days, which is why a box for low mood is a conversation with a physician and not a purchase.34,35,36,47Grade B

  • A week, or a weekend, without electric light

    resets the clock to the sun within days, most for the latest sleepers

    A week of camping in summer moved every marker of internal time about 2 hours earlier in 8 adults, set melatonin onset near sunset and offset before waking, and advanced the latest chronotypes most, by 0.83 hours for every hour later their baseline chronotype sat; weekly sleep duration did not change, so the shift was light, not sleep. A winter week did the same and widened the biological night, and a single summer weekend of camping delivered about 69 percent of the week's shift. The point is not the tent; it is that the clock is still ready to follow the sun when it is allowed to, and that the first two hours of daylight and the last three of darkness are where the difference was made.11,12Grade C

  • Shift work and the inverted clock

    the strongest misalignment; higher diabetes odds and a probable carcinogen classification, with no proven fix

    Ten days of eating and sleeping 12 hours out of phase raised glucose 6 percent despite 22 percent more insulin, reversed the cortisol rhythm and put 3 of 8 volunteers into a prediabetic post-meal range; in the UK Biobank, usual night shifts carried odds of 1.44 for type 2 diabetes and more than 8 night shifts a month over a lifetime 1.36; IARC classes night shift work as probably carcinogenic (Group 2A). The physiology suggests light during the shift, darkness after it and as regular a pattern as the roster allows, but no trial has shown that managing light lowers a shift worker's risk, and the page says so.32,41,42,43Grade B

07 · Reading it well

Caveats, and what belongs with a physician.

The mortality evidence is observational, measured at the wrist and recorded for one week. The UK Biobank light cohort was 97 percent white and 62 years old on average, its sensors read ordinary lux at the wrist rather than melanopic lux at the eye, a sleeve or a blanket reads as darkness, and two of its senior authors report industry consulting and research funding, including from lighting companies. The hazards survived adjustment for shift work, physical activity, deprivation and baseline cardiometabolic health, and the dose-response in both directions is what earns the B grade, but a brighter night may still mark illness, pain, a partner's schedule or a city flat as much as cause harm, and for cause-specific mortality the day-light association weakened at the fullest adjustment, leaving only the brightest tenth of days significant.23Grade B

Light at the eye is almost never measured. The half-million-person mood cohort asked people how many hours they spent outdoors, the Sister Study asked women what light they slept with, and the Chinese diabetes study assigned every participant at a site the satellite radiance over that site; the consensus panel's own figures are written in a unit that requires a calibrated meter held at eye level. Every cohort on this page is therefore reading a proxy, and the proxies disagree with each other by orders of magnitude.21,27,29,7Grade C

The light-therapy trials are short and the treatment has side effects. The nonseasonal trial ran 8 weeks and the bipolar trial 6; the safety review asks clinicians to watch for emergent hypomania and autonomic over-activation in the first days, and the bipolar trial achieved its result with midday rather than morning light, in people on stable antimanic medication, after excluding anyone with current hypomania, mixed symptoms or rapid cycling. A light box in someone with a bipolar history, or in anyone whose low mood has not been assessed, is a prescription-level decision.35,36,47Grade B

This entry asks for daylight at the eye, not sun on the skin. A bright morning can be had under a hat or in the shade, and nothing here requires a burn. Many common medications sensitize the skin to ultraviolet or visible light, among the most consistently implicated amiodarone, doxycycline and tetracycline, hydrochlorothiazide, naproxen and piroxicam, and voriconazole, and prevention, meaning sun protection and knowing the risk, is the mainstay of management; some reactions carry a longer-term skin cancer risk at the affected sites. The eye matters too: the circadian pathway can be intact in a person who cannot see, and retinal disease, recent eye surgery or an eye condition under treatment are reasons to ask an ophthalmologist before using a light box, a judgment this page states as one.48,3,47Grade D

The same light is a different dose to different people. The individual threshold for half-maximal melatonin suppression ranged from 6 to 350 lux across 55 young adults, and the consensus panel notes that the aging lens roughly halves the retinal dose of a given melanopic lux relative to the young standard observer. A rule written for the average eye will over-dose the most sensitive and under-dose the oldest, and no clinical test yet tells a person which they are.14,7Grade C

Melatonin suppression is not the whole mechanism, and the mechanisms are not settled. In the one-night bedroom light trial, melatonin did not differ between 100 lux and under 3 lux, yet insulin resistance rose and sympathetic tone during sleep tracked it; the blue-light metabolic study calls its own mechanism unclear; and the mood pathway through the perihabenular nucleus has been shown in mice, not people. The page reports what moved, and does not claim to know every reason why.30,31,20Grade C

See a physician

  • Low mood that returns each autumn or winter, or any low mood with loss of interest for more than two weeks, before buying a light box; and any thought of self-harm, which is urgent and not a lighting question.
  • A diagnosis or family history of bipolar disorder, before starting bright light at any hour.
  • Eye disease of any kind, recent eye surgery, new difficulty seeing, or eye pain, before using a light box; an ophthalmologist decides, not a page.
  • A rash on sun-exposed skin while taking a medication, which can be drug-induced photosensitivity and may need the drug changed.
  • Being unable to fall asleep until the early hours and unable to wake at a normal time for months, or falling asleep in the early evening and waking in the small hours, both of which are circadian rhythm disorders with specific treatments.
  • Shift work with sleepiness at the wheel, near misses, or falling asleep against your will during the day.
  • Blindness or severe visual loss with insomnia that comes and goes in cycles of weeks, which can be a free-running clock and is treatable.

This page is education, not individual medical advice, and reading it creates no physician-patient relationship.

08 · Open questions

What the literature does not settle.

  • No randomized trial has tested morning light, dim evenings or a dark bedroom against a hard outcome: heart attack, diabetes diagnosis, dementia or death. The mortality and diabetes evidence is observational; the trials measure melatonin, insulin, mood and sleep over nights to weeks.
  • There is no evidence-based dose in hours. The consensus figures are indoor-lighting targets built from laboratory physiology, the one large outdoor-hours cohort found a graded association with no threshold, and how many minutes outdoors, at what brightness and in which hour, is a question nobody has randomized.
  • Light has not been measured at the eye in any large cohort. Whether wrist sensors, self-reported hours and satellite radiance are even measuring the same thing is unknown, and the consensus's melanopic unit has yet to be worn by a population.
  • Whether a bright day protects against a lit night is untested. The mortality cohort's authors note that daytime light alters the clock's sensitivity to light at night, and the laboratory dose-response was measured in people kept in dim light beforehand.
  • Why some people register 6 lux as half a dose and others need 350 is not understood, and no clinical test sorts the sensitive from the robust; the aging lens is one known factor and genetics another.
  • Whether screens matter through their light, their content, or the hour they keep people awake has not been separated in a trial with outcomes beyond the next morning.
  • Whether managing light lowers the diabetes, cardiovascular or cancer risk of shift workers, in whom the misalignment is largest, has not been shown.

09 · In practice

How Dr. Tagge reads it in his own practice.

I treat light as the setting every hormone on this shelf is read against, and I ask about it before I ask about supplements. What time do you wake, and is it the same on Saturday as on Tuesday? Where are you for the first two hours of the day? What does the room look like at ten at night, and what is on in the bedroom? I do not give a minutes target or a lux target, because the evidence does not give me one. The only figures I will quote are the consensus's, at least 250 melanopic lux through the day indoors, no more than 10 in the three hours before bed and no more than 1 while asleep, and those were written for lighting designers, not for patients. What I ask for is simpler: a fixed wake time, outside soon after it, a dim evening, a dark room. That is the first move for sleep, before melatonin, before magnesium, before anything in a bottle. Light is also where I read cortisol: a flat morning cortisol in someone who wakes in a dark room and sits under a desk lamp until noon is not a gland problem until the mornings have been fixed. The same goes for a fasting glucose that creeps and an insulin that climbs in someone who eats late under bright light and falls asleep to a screen; one bright night moves next-morning insulin resistance in the trials, and I want that variable out before I decide what the number means. One marker is a clue, three are a story, and light touches all three. Shift workers get a different conversation. I cannot give them a day that does not exist, so we protect what we can, light during the shift, darkness after it, as regular a pattern as the roster allows, and I say honestly that the cohorts show higher risk however well it is managed. With screens I care about brightness and the hour more than the device. I will say plainly where I am ahead of the evidence. The mortality data are observational and were worn on the wrist, the light-therapy trials run eight weeks, and nobody has randomized morning walks against heart attacks. I act on the physiology, the cohorts and the fact that this lever carries almost no risk, and I say so.

A practice judgment, labeled as one (Grade D): it is how one physician reads the number for the people he cares for, not a recommendation for you.

10

Questions

11 · Sources

48 sources, read in full.

Numbered in the order they appear. Each line says what this page relies on from the paper; the link opens the record at PubMed or the publisher.

  1. 1.

    Berson DM, Dunn FA, Takao M. Phototransduction by retinal ganglion cells that set the circadian clock. Science. 2002. PMID 11834835. DOI 10.1126/science.1067262.

    Laboratory method study · Retinal ganglion cells innervating the suprachiasmatic nucleus are intrinsically photosensitive, depolarizing to light with all rod and cone input blocked; their sensitivity, spectral tuning and slow kinetics match the photic entrainment mechanism, making them candidate primary photoreceptors for the circadian system.

  2. 2.

    Hattar S, Liao HW, Takao M, Berson DM, Yau KW. Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science. 2002. PMID 11834834. DOI 10.1126/science.1069609.

    Laboratory method study · Melanopsin is present in a subset of rat retinal ganglion cells whose axons project to the suprachiasmatic nucleus and other nuclei of circadian photoentrainment and the pupillary reflex; every intrinsically photosensitive ganglion cell expressed melanopsin, making it the likely pigment of the cells that set the clock.

  3. 3.

    Czeisler CA, Shanahan TL, Klerman EB, et al.. Suppression of melatonin secretion in some blind patients by exposure to bright light. New England Journal of Medicine. 1995. PMID 7990870. DOI 10.1056/NEJM199501053320102.

    Laboratory method study · Among 11 blind patients with no conscious light perception, bright light lowered plasma melatonin by a mean 69 percent in 3 (66 percent in 6 sighted controls), not with eyes covered; those 3 slept normally and stayed entrained to 24 hours, while 7 of the remaining 8 showed no suppression and reported insomnia, 4 with unentrained temperature rhythms.

  4. 4.

    Czeisler CA, Duffy JF, Shanahan TL, et al.. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. 1999. PMID 10381883. DOI 10.1126/science.284.5423.2177.

    Laboratory method study · Under carefully controlled lighting, the intrinsic period of the human circadian pacemaker measured from melatonin, core body temperature and cortisol averaged 24.18 hours in both young and older adults, with a tight distribution like other species.

  5. 5.

    Khalsa SB, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. Journal of Physiology. 2003. PMID 12717008. DOI 10.1113/jphysiol.2003.040477.

    Laboratory method study · In 21 healthy adults, a 6.7-hour bright light pulse (about 10,000 lux) produced a phase response curve with a 5.02-hour peak-to-trough span: delays before the core body temperature minimum, advances after it, and no dead zone during the day.

  6. 6.

    Lockley SW, Brainard GC, Czeisler CA. High sensitivity of the human circadian melatonin rhythm to resetting by short wavelength light. Journal of Clinical Endocrinology and Metabolism. 2003. PMID 12970330. DOI 10.1210/jc.2003-030570.

    Laboratory method study · 6.5 hours of monochromatic 460 nm light produced a twofold greater circadian phase delay and twice the melatonin suppression of 555 nm light at equal photon density; the pacemaker's peak sensitivity is blue-shifted from the 555 nm photopic peak, so photopic lux is inappropriate for quantifying circadian light.

  7. 7.

    Brown TM, Brainard GC, Cajochen C, et al.. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biology. 2022. PMID 35298459. DOI 10.1371/journal.pbio.3001571.

    Consensus statement · Expert consensus from a 2019 workshop of 18 researchers: through the daytime a minimum of 250 melanopic equivalent daylight illuminance (lux) at the eye in the vertical plane at about 1.2 m, daylight first; from at least 3 hours before bedtime a maximum of 10 melanopic lux; during sleep a maximum of 1 melanopic lux, 10 if vision is needed; a regular light-dark cycle likely reinforces alignment; the aging lens roughly halves retinal dose; typical evening indoor illuminance about 30 photopic lux.

  8. 8.

    Lewy AJ, Wehr TA, Goodwin FK, Newsome DA, Markey SP. Light suppresses melatonin secretion in humans. Science. 1980. PMID 7434030. DOI 10.1126/science.7434030.

    Laboratory method study · Bright artificial light suppressed nighttime melatonin secretion in six healthy subjects while ordinary room light did not, establishing that the human response to light is qualitatively like that of other mammals.

  9. 9.

    Brainard GC, Hanifin JP, Greeson JM, et al.. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. Journal of Neuroscience. 2001. PMID 11487664. DOI 10.1523/JNEUROSCI.21-16-06405.2001.

    Laboratory method study · In 72 healthy adults tested 627 times with monochromatic light from 420 to 600 nm between 2 and 3:30 AM, melatonin suppression fit an opsin template identifying 446 to 477 nm as the most potent region, distinct from rod and cone pigments, evidence of a novel circadian photopigment.

  10. 10.

    Thapan K, Arendt J, Skene DJ. An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. Journal of Physiology. 2001. PMID 11507175. DOI 10.1111/j.1469-7793.2001.t01-1-00261.x.

    Laboratory method study · In 22 volunteers and 215 trials of 30-minute monochromatic light at 424 to 548 nm, melatonin suppression showed short-wavelength sensitivity unlike rods or cones, best fit by a rhodopsin template peaking at 459 nm: the first direct evidence of a non-rod, non-cone photoreceptive system in humans.

  11. 11.

    Wright KP Jr, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED. Entrainment of the human circadian clock to the natural light-dark cycle. Current Biology. 2013. PMID 23910656. DOI 10.1016/j.cub.2013.06.039.

    Laboratory method study · Eight adults averaged 979 lux across waking hours in the built environment and 4,487 lux during a week of summer camping without electric light (3,074 against 934 lux in the first 2 hours awake); after the camping week all markers of internal time were about 2 hours earlier, melatonin onset fell near sunset and offset before waking, later chronotypes advanced most (0.83 hours per hour of chronotype), and sleep duration did not change.

  12. 12.

    Stothard ER, McHill AW, Depner CM, et al.. Circadian Entrainment to the Natural Light-Dark Cycle across Seasons and the Weekend. Current Biology. 2017. PMID 28162893. DOI 10.1016/j.cub.2016.12.041.

    Laboratory method study · A week in a natural winter 9 hour 20 minute light-dark cycle moved the biological night and sleep earlier than the electric-lighting environment and expanded the biological night relative to summer; a single summer weekend of camping moved circadian and sleep timing earlier and achieved about 69 percent of the shift previously seen after a full week.

  13. 13.

    Espiritu RC, Kripke DF, Ancoli-Israel S, et al.. Low illumination experienced by San Diego adults: association with atypical depressive symptoms. Biological Psychiatry. 1994. PMID 8018787. DOI 10.1016/0006-3223(94)90007-8.

    Cross-sectional study · Among 106 randomly recruited San Diego adults aged 40 to 64 wearing an illumination monitor, the median subject was at or above 1,000 lux for only 4 percent of the time observed, about 58 minutes a day; more atypical seasonal mood symptoms went with less time in bright light (rs = minus 0.27).

  14. 14.

    Phillips AJK, Vidafar P, Burns AC, et al.. High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proceedings of the National Academy of Sciences of the United States of America. 2019. PMID 31138694. DOI 10.1073/pnas.1901824116.

    Laboratory method study · In 55 adults aged 18 to 30 exposed to 5 hours of evening light at 10 to 2,000 lux against under 1 lux, the group dose for 50 percent melatonin suppression was 24.6 lux; 10, 30 and 50 lux delayed apparent melatonin onset by 22, 77 and 109 minutes; individual half-maximal doses ranged from 6 to 350 lux; before electric light humans lived mostly above 300 lux or below 30, and now spend evening hours at 30 to 300.

  15. 15.

    Gooley JJ, Chamberlain K, Smith KA, et al.. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. Journal of Clinical Endocrinology and Metabolism. 2011. PMID 21193540. DOI 10.1210/jc.2010-2098.

    Laboratory method study · In 116 healthy adults aged 18 to 30, room light under 200 lux in the 8 hours before bedtime, against dim light under 3 lux, delayed melatonin onset in 99 percent and shortened melatonin duration by about 90 minutes; room light during the usual sleep hours suppressed melatonin by more than 50 percent in 85 percent of trials.

  16. 16.

    Scheer FA, Buijs RM. Light affects morning salivary cortisol in humans. Journal of Clinical Endocrinology and Metabolism. 1999. PMID 10487717. DOI 10.1210/jcem.84.9.6102.

    Laboratory method study · In 14 healthy men, 800 lux for one hour after waking raised the awakening cortisol peak about 35 percent above darkness, with higher levels at 20 and 40 minutes after waking; the same light in the late evening had no effect on cortisol.

  17. 17.

    Leproult R, Colecchia EF, L'Hermite-Balériaux M, Van Cauter E. Transition from dim to bright light in the morning induces an immediate elevation of cortisol levels. Journal of Clinical Endocrinology and Metabolism. 2001. PMID 11231993. DOI 10.1210/jcem.86.1.7102.

    Laboratory method study · In 8 healthy men kept awake 36 hours, 3 hours of bright light from 5 to 8 AM suppressed melatonin, raised cortisol immediately by more than 50 percent and limited the overnight loss of alertness, with no effect on TSH; the same exposure from 1 to 4 PM changed nothing.

  18. 18.

    Cajochen C, Münch M, Kobialka S, et al.. High sensitivity of human melatonin, alertness, thermoregulation, and heart rate to short wavelength light. Journal of Clinical Endocrinology and Metabolism. 2005. PMID 15585546. DOI 10.1210/jc.2004-0957.

    Laboratory method study · Two hours of monochromatic 460 nm light in the late evening suppressed melatonin more than 550 nm light at equal photon density, with a greater alerting response and increased core body temperature and heart rate; the alerting and thermoregulatory responses are blue-shifted like the circadian ones.

  19. 19.

    Morris CJ, Yang JN, Garcia JI, et al.. Endogenous circadian system and circadian misalignment impact glucose tolerance via separate mechanisms in humans. Proceedings of the National Academy of Sciences of the United States of America. 2015. PMID 25870289. DOI 10.1073/pnas.1418955112.

    Laboratory method study · In healthy adults in two 8-day protocols, post-meal glucose was 17 percent higher in the biological evening (8 PM) than the biological morning (8 AM) independent of the behavioral cycle, through 27 percent lower early-phase insulin; circadian misalignment itself raised post-meal glucose a further 6 percent despite 14 percent higher late-phase insulin, consistent with lower insulin sensitivity.

  20. 20.

    Fernandez DC, Fogerson PM, Lazzerini Ospri L, et al.. Light Affects Mood and Learning through Distinct Retina-Brain Pathways. Cell. 2018. PMID 30173913. DOI 10.1016/j.cell.2018.08.004.

    Laboratory method study · In mice under a 3.5-hour light-dark cycle, light's effects on learning ran through melanopsin ganglion cells projecting to the suprachiasmatic nucleus, independent of its pacemaker function, while its effects on mood required a separate pathway from those cells to a thalamic region named the perihabenular nucleus, necessary and sufficient for the affective effect.

  21. 21.

    Burns AC, Saxena R, Vetter C, Phillips AJK, Lane JM, Cain SW. Time spent in outdoor light is associated with mood, sleep, and circadian rhythm-related outcomes: A cross-sectional and longitudinal study in over 400,000 UK Biobank participants. Journal of Affective Disorders. 2021. PMID 34488088. DOI 10.1016/j.jad.2021.08.056.

    Prospective cohort · UK Biobank participants reported a median 2.5 daylight hours outdoors a day (IQR 1.5 to 3.5); each additional hour was associated with lower odds of lifetime major depression (OR CI 0.92 to 0.98), antidepressant use, anhedonia and low mood, greater happiness (1.41 to 1.48), easier getting up (1.46 to 1.49), less tiredness (0.80 to 0.82), fewer insomnia symptoms (0.94 to 0.97) and earlier chronotype (0.75 to 0.77); longitudinal models agreed with smaller effects.

  22. 22.

    Windred DP, Burns AC, Lane JM, et al.. Sleep regularity is a stronger predictor of mortality risk than sleep duration: A prospective cohort study. Sleep. 2024. PMID 37738616. DOI 10.1093/sleep/zsad253.

    Prospective cohort · In 60,977 UK Biobank adults with accelerometer-derived Sleep Regularity Index scores and 1,859 deaths over a mean 6.3 years, the four more regular quintiles had 20 to 48 percent lower all-cause mortality than the least regular; regularity predicted mortality more strongly than duration.

  23. 23.

    Windred DP, Burns AC, Lane JM, et al.. Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals. Proceedings of the National Academy of Sciences of the United States of America. 2024. PMID 39405349. DOI 10.1073/pnas.2405924121.

    Prospective cohort · In 88,905 UK Biobank adults wearing wrist light sensors for 7 days and followed a mean 8.0 years (3,750 deaths), the 70th to 90th and brightest tenth of night light carried 15 to 17 and 21 to 34 percent higher all-cause mortality, and the 50th to 70th, 70th to 90th and brightest tenth of day light 10 to 16, 16 to 26 and 17 to 34 percent lower, against the darker half; cardiometabolic deaths carried larger hazards; lower modeled circadian amplitude and early or late phase also predicted mortality.

  24. 24.

    Burns AC, Windred DP, Rutter MK, et al.. Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people. Nature Mental Health. 2023. DOI 10.1038/s44220-023-00135-8.

    Cross-sectional study · In 86,772 UK Biobank adults (mean age 62.4, 57 percent women) with sensor-measured light, greater night light was associated with higher risk of major depressive disorder, generalized anxiety disorder, PTSD, psychosis, bipolar disorder and self-harm, and greater daytime light, independently, with lower risk of major depression, PTSD, psychosis and self-harm, robust to adjustment for sociodemographics, photoperiod, physical activity, sleep quality and cardiometabolic health; not indexed in PubMed, abstract only read.

  25. 25.

    Obayashi K, Saeki K, Kurumatani N. Bedroom Light Exposure at Night and the Incidence of Depressive Symptoms: A Longitudinal Study of the HEIJO-KYO Cohort. American Journal of Epidemiology. 2018. PMID 28992236. DOI 10.1093/aje/kwx290.

    Prospective cohort · Among 863 older Japanese adults (mean age 71.5) without depressive symptoms at baseline, 73 developed them over a median 24 months; bedrooms averaging 5 lux or more at night (n = 153) carried a hazard ratio of 1.89 (1.13 to 3.14), 1.72 (1.03 to 2.89) after adjustment for hypertension, diabetes and sleep, consistent at a 10 lux cutoff.

  26. 26.

    Obayashi K, Yamagami Y, Kurumatani N, Saeki K. Bedroom lighting environment and incident diabetes mellitus: a longitudinal study of the HEIJO-KYO cohort. Sleep Medicine. 2020. PMID 31704511. DOI 10.1016/j.sleep.2019.07.006.

    Prospective cohort · Among 678 older adults without diabetes, 19 developed it over a median 42 months; bedrooms averaging 5 lux or more at night (n = 128) carried an incidence rate ratio of 3.74 (1.55 to 9.05), 3.19 after propensity adjustment and 2.74 at a 3 lux cutoff.

  27. 27.

    Park YM, White AJ, Jackson CL, Weinberg CR, Sandler DP. Association of Exposure to Artificial Light at Night While Sleeping With Risk of Obesity in Women. JAMA Internal Medicine. 2019. PMID 31180469. DOI 10.1001/jamainternmed.2019.0571.

    Prospective cohort · Among 43,722 women aged 35 to 74 (no shift workers) followed a mean 5.7 years, any light during sleep carried RR 1.19 for incident obesity, and sleeping with a television or light on in the room carried RR 1.17 for gaining 5 kg or more, 1.13 for a 10 percent BMI rise, 1.22 for incident overweight and 1.33 for incident obesity against no light, robust to adjustment for sleep, diet and activity.

  28. 28.

    Reid KJ, Santostasi G, Baron KG, Wilson J, Kang J, Zee PC. Timing and intensity of light correlate with body weight in adults. PLoS One. 2014. PMID 24694994. DOI 10.1371/journal.pone.0092251.

    Cross-sectional study · In 54 adults wearing a wrist light recorder for 7 days, the mean clock time of light above 500 lux correlated with BMI (r = 0.51) and remained an independent predictor after adjustment for sleep midpoint, duration and covariates, explaining about 20 percent of the variance.

  29. 29.

    Zheng R, Xin Z, Li M, et al.. Outdoor light at night in relation to glucose homoeostasis and diabetes in Chinese adults: a national and cross-sectional study of 98,658 participants from 162 study sites. Diabetologia. 2023. PMID 36372821. DOI 10.1007/s00125-022-05819-x.

    Cross-sectional study · Among 98,658 Chinese adults at 162 sites, satellite-estimated outdoor light at night was associated with higher HbA1c, fasting and 2-hour glucose and HOMA-IR and lower HOMA-B; diabetes prevalence ratio 1.07 per quintile and 1.28 (1.03 to 1.60) for the brightest fifth against the darkest, adjusted for BMI and lifestyle.

  30. 30.

    Mason IC, Grimaldi D, Reid KJ, et al.. Light exposure during sleep impairs cardiometabolic function. Proceedings of the National Academy of Sciences of the United States of America. 2022. PMID 35286195. DOI 10.1073/pnas.2113290119.

    Randomized trial · Twenty healthy young adults randomized to one night of 100 lux overhead room light during sleep (n = 10) or a second dim night under 3 lux (n = 10): HOMA-IR rose about 15 percent against a 4 percent fall, the Matsuda index fell about 16 percent against a 3 percent rise, 30-minute insulin area was higher, heart rate higher and heart rate variability lower during sleep, melatonin unchanged; higher sympathovagal balance tracked higher insulin.

  31. 31.

    Cheung IN, Zee PC, Shalman D, Malkani RG, Kang J, Reid KJ. Morning and Evening Blue-Enriched Light Exposure Alters Metabolic Function in Normal Weight Adults. PLoS One. 2016. PMID 27191727. DOI 10.1371/journal.pone.0155601.

    Randomized trial · In 19 healthy adults living 4 days in dim light (under 20 lux), 3 hours of blue-enriched light starting 0.5 or 10.5 hours after waking raised insulin total area, HOMA-IR and HOMA-IR area and reduced sleepiness against the dim baseline; the evening group also had a higher glucose peak; mechanism called unclear.

  32. 32.

    Scheer FA, Hilton MF, Mantzoros CS, Shea SA. Adverse metabolic and cardiovascular consequences of circadian misalignment. Proceedings of the National Academy of Sciences of the United States of America. 2009. PMID 19255424. DOI 10.1073/pnas.0808180106.

    Laboratory method study · Ten adults living a 28-hour day for 10 days showed, when about 12 hours out of phase, leptin 17 percent lower, glucose 6 percent higher despite insulin 22 percent higher, a completely reversed cortisol rhythm, mean arterial pressure 3 percent higher and sleep efficiency 20 percent lower; 3 of 8 reached prediabetic post-meal glucose.

  33. 33.

    Zeitzer JM, Dijk DJ, Kronauer R, Brown E, Czeisler C. Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. Journal of Physiology. 2000. PMID 10922269. DOI 10.1111/j.1469-7793.2000.00695.x.

    Laboratory method study · In 23 healthy young adults given a single 6.5-hour light exposure in the early biological night, phase delay and melatonin suppression followed logistic dose-response curves, and half of the maximal delay produced by about 9,000 lux was obtained with just over 1 percent of it, about 100 lux of dim room light.

  34. 34.

    Golden RN, Gaynes BN, Ekstrom RD, et al.. The efficacy of light therapy in the treatment of mood disorders: a review and meta-analysis of the evidence. American Journal of Psychiatry. 2005. PMID 15800134. DOI 10.1176/appi.ajp.162.4.656.

    Systematic review and meta-analysis · Across randomized trials to 2003, bright light reduced depression severity in seasonal affective disorder (8 studies, effect size 0.84, 0.60 to 1.08), as did dawn simulation (5 studies, 0.73) and bright light in nonseasonal depression (3 studies, 0.53, 0.18 to 0.89); bright light as an adjunct to antidepressants in nonseasonal depression was not effective (5 studies, minus 0.01); only 13 percent of studies met inclusion criteria.

  35. 35.

    Lam RW, Levitt AJ, Levitan RD, et al.. Efficacy of Bright Light Treatment, Fluoxetine, and the Combination in Patients With Nonseasonal Major Depressive Disorder: A Randomized Clinical Trial. JAMA Psychiatry. 2016. PMID 26580307. DOI 10.1001/jamapsychiatry.2015.2235.

    Randomized trial · In 122 adults with nonseasonal major depression randomized for 8 weeks, MADRS fell 13.4 with a 10,000 lux light box for 30 minutes each early morning, 8.8 with fluoxetine 20 mg, 16.9 with both and 6.5 with placebo; light (d = 0.80) and combination (1.11) beat placebo, fluoxetine alone (0.24) did not; remission 43.8, 19.4, 58.6 and 30.0 percent; all well tolerated.

  36. 36.

    Sit DK, McGowan J, Wiltrout C, et al.. Adjunctive Bright Light Therapy for Bipolar Depression: A Randomized Double-Blind Placebo-Controlled Trial. American Journal of Psychiatry. 2018. PMID 28969438. DOI 10.1176/appi.ajp.2017.16101200.

    Randomized trial · In 46 adults with bipolar I or II depression on stable antimanic medication (23 per group), 6 weeks of 7,000 lux midday white light produced remission in 68.2 percent against 22.2 percent with 50 lux dim red light (adjusted OR 12.6), lower depression scores, and no mood polarity switches in either group.

  37. 37.

    Wittmann M, Dinich J, Merrow M, Roenneberg T. Social jetlag: misalignment of biological and social time. Chronobiology International. 2006. PMID 16687322. DOI 10.1080/07420520500545979.

    Cross-sectional study · In 501 volunteers completing the Munich ChronoType Questionnaire, late chronotypes showed the largest work-to-free-day differences in sleep timing; associations of chronotype with poor sleep quality, low wellbeing and stimulant use, strongest in people under 25 and most striking for smoking, were attributed to social jetlag rather than chronotype itself.

  38. 38.

    Roenneberg T, Allebrandt KV, Merrow M, Vetter C. Social jetlag and obesity. Current Biology. 2012. PMID 22578422. DOI 10.1016/j.cub.2012.03.038.

    Cross-sectional study · In a large-scale epidemiological survey, social jetlag, the discrepancy between circadian and social clocks, was associated with higher body mass index beyond the effect of sleep duration; the authors read living against the clock as a contributor to obesity.

  39. 39.

    Parsons MJ, Moffitt TE, Gregory AM, et al.. Social jetlag, obesity and metabolic disorder: investigation in a cohort study. International Journal of Obesity. 2015. PMID 25601363. DOI 10.1038/ijo.2014.201.

    Prospective cohort · Among 815 non-shift-working adults of the Dunedin cohort, social jetlag was associated with clinically assessed measures of metabolic dysfunction and obesity, was higher in metabolically unhealthy than metabolically healthy obese participants, and among the former was associated with higher glycated hemoglobin and an inflammatory marker.

  40. 40.

    Walch OJ, Cochran A, Forger DB. A global quantification of "normal" sleep schedules using smartphone data. Science Advances. 2016. PMID 27386531. DOI 10.1126/sciadv.1501705.

    Cross-sectional study · From a smartphone app's worldwide sleep data, social pressures delay bedtime and shorten sleep while wake time is less affected; users reporting typical exposure to outdoor light went to sleep earlier and slept more than those reporting indoor light; women scheduled more sleep than men, and age was the primary determinant of sleep timing.

  41. 41.

    Vetter C, Dashti HS, Lane JM, et al.. Night Shift Work, Genetic Risk, and Type 2 Diabetes in the UK Biobank. Diabetes Care. 2018. PMID 29440150. DOI 10.2337/dc17-1933.

    Cross-sectional study · Among 272,214 UK Biobank participants, current night shift workers had higher adjusted odds of type 2 diabetes than day workers (none or rare nights 1.15, some nights 1.18, usual nights 1.44; permanent nights 1.09, not significant); among 70,480 with lifetime schedules, more than 8 night shifts a month carried OR 1.36 against never, with a significant trend; shift work did not modify a 110-variant genetic risk score.

  42. 42.

    IARC Monographs Vol 124 group. Carcinogenicity of night shift work. Lancet Oncology. 2019. PMID 31281097. DOI 10.1016/S1470-2045(19)30455-3.

    Consensus statement · The IARC Monographs working group's summary evaluation of night shift work, published July 2019; its classification wording is quoted here through Erren 2019 because the summary carries no abstract.

  43. 43.

    Erren TC, Morfeld P, Groß JV, et al.. IARC 2019: "Night shift work" is probably carcinogenic: What about disturbed chronobiology in all walks of life?. Journal of Occupational Medicine and Toxicology. 2019. PMID 31798667. DOI 10.1186/s12995-019-0249-6.

    Review · Reports that in June 2019 an IARC working group of 27 experts from 16 countries classified night shift work as probably carcinogenic to humans (Group 2A), on sufficient evidence in experimental animals, strong mechanistic evidence and limited human evidence, the same weakest stream as in 2007.

  44. 44.

    Figueiro MG, Steverson B, Heerwagen J, et al.. The impact of daytime light exposures on sleep and mood in office workers. Sleep Health. 2017. PMID 28526259. DOI 10.1016/j.sleh.2017.03.005.

    Cross-sectional study · Among 109 office workers in five US federal buildings wearing a calibrated circadian-light meter for 7 days (81 in both winter and summer), high circadian-effective light in the morning was associated with shorter sleep onset latency, especially in winter, stronger entrainment and better sleep quality; high levels across the whole day with stronger entrainment, less depression and better sleep quality.

  45. 45.

    Boubekri M, Cheung IN, Reid KJ, Wang CH, Zee PC. Impact of windows and daylight exposure on overall health and sleep quality of office workers: a case-control pilot study. Journal of Clinical Sleep Medicine. 2014. PMID 24932139. DOI 10.5664/jcsm.3780.

    Cross-sectional study · Among 49 office workers, the 27 in windowless workplaces scored worse than the 22 with windows on vitality, role limitation due to physical problems and overall sleep quality; in the 21 with actigraphy, workers with windows had more light exposure during the workweek, a trend to more physical activity and longer sleep.

  46. 46.

    Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proceedings of the National Academy of Sciences of the United States of America. 2015. PMID 25535358. DOI 10.1073/pnas.1418490112.

    Randomized trial · Reading a light-emitting e-book before bed, versus a printed book, lengthened time to fall asleep, reduced evening sleepiness, suppressed melatonin, delayed the circadian clock and lowered next-morning alertness; 90 percent of 1,508 surveyed Americans used electronics within an hour of bedtime at least a few nights a week.

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    Terman M, Terman JS. Light therapy for seasonal and nonseasonal depression: efficacy, protocol, safety, and side effects. CNS Spectrums. 2005. PMID 16041296. DOI 10.1017/s1092852900019611.

    Review · Bright light therapy is an established treatment for seasonal affective disorder and extends to nonseasonal unipolar and bipolar depression and circadian sleep phase disorders; dosing and time of day need individual adjustment; the side-effect profile is favorable but clinicians must watch for emergent hypomania and autonomic hyperactivation, especially in the first days.

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    Blakely KM, Drucker AM, Rosen CF. Drug-Induced Photosensitivity-An Update: Culprit Drugs, Prevention and Management. Drug Safety. 2019. PMID 30888626. DOI 10.1007/s40264-019-00806-5.

    Review · Photosensitive drug eruptions follow a medication plus ultraviolet or visible light on sun-exposed skin; prevention through informing patients and sun protection is the mainstay; amiodarone, chlorpromazine, doxycycline, hydrochlorothiazide, nalidixic acid, naproxen, piroxicam, tetracycline, thioridazine, vemurafenib and voriconazole are the most consistently implicated, and some reactions warrant long-term skin cancer surveillance.

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