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Reference topic 23: Sleep and the Dark You Have Lost

Light at night and sleep: blue light, darkness, body clock

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.80 sources5 graded recommendationsTopic hub: Sleep and light

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On 16 July 1962 a 23-year-old French geologist climbed down into a cave in the Alps, about 70 kilometers from Nice, where his team had found an underground glacier the year before. He stayed about two months.1 Michel Siffre's camp sat more than 100 meters down, on ice, below freezing, at 98% humidity, with a tent, a butane stove that could poison the air, and loose rock overhead.23 "My feet were always wet," he said later, "and my body temperature got as low as 34°C (93°F)."1

No watch, no calendar, no daylight. What he had was a telephone line to a team at the entrance. "I decided I would call them when I woke up, when I ate, and just before I went to sleep."1 Time came loose. Asked to count to 120 at a number a second, he took five minutes: "I psychologically experienced five real minutes as though they were two."1

Mismatch card

The system

The circadian clock, set through melanopsin-carrying retinal cells, and the sleep it schedules.

What it expects

A bright day, a dark night, a falling evening temperature, and the same schedule tomorrow.

What it gets

Days under 100 lux indoors, evenings at 30 to 300 lux, a lit bedroom, a schedule that moves at weekends.

What happens in the gap

Sleep moves later and grows irregular, the melatonin night shortens, and peripheral clocks fall out of step.

The handrail

Bright mornings outdoors, dim evenings, a dark bedroom, regular timing (●●○)

Strength of evidence

●●○ Light's effect on clock timing is proven in trials, but the link from lit nights to disease and death rests on observational cohorts.

What this topic covers

  • ·The Clock Beneath the Clock
  • ·A Sense You Did Not Know You Had
  • ·The Price of Light
  • ·The Days That Are Too Dark
  • ·How People Without Lamps Actually Sleep
  • ·The Clock That Rings
  • ·Brighter Days, Darker Nights

The practical version is in the book: Chapter 8, Sleep.

The handrail

Topic 41 holds the full program.

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Get outside early

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Daylight in the first hours after waking, and at least 250 melanopic lux at the eye through the day (light weighted for its effect on melanopsin).38 Morning light advances the clock and evening light delays it.75 Measured daylight tracks better mood, sleep and survival, though observationally.3764

  2. Dim the last three hours, and sleep in the dark

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for timing·Evidence grade: Emerging. Early, small, animal-only, or actively contested. for long-term health

    Below about 10 melanopic lux in the evening and 1 in the bedroom.3866 Sensitivity varies fiftyfold, so judge by your own sleep.29 If you get up at night, use a dim night light rather than risk a fall.

  3. Keep the timing regular, weekends included

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Regularity predicted mortality better than duration in 60,977 people.63 Adults should aim for seven hours or more.76 A weekend outdoors pulls a late clock earlier (●○○).41

  4. Change screen habits, not lenses

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for children and teens·Evidence grade: Emerging. Early, small, animal-only, or actively contested. for adults

    A device out of the bed and a cut-off time have better support than filters or blue-blocking glasses.707172 Lower screen brightness does cut melatonin suppression in the laboratory.77 Stop caffeine at least six hours before bed: 400 mg that late still disturbed sleep in a randomized trial.78

  5. For insomnia, start with what works

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    The American College of Physicians recommends cognitive behavioral therapy as the first treatment for chronic insomnia.79 Melatonin is not recommended for it in adults.80 Gummy content is unreliable, from 74% to 347% of the label, so keep melatonin away from children.80 Snoring, gasping, or sleepiness that persists despite enough time in bed is worth a clinician's attention (Topic 16). Never stop prescribed sleep medicine abruptly without medical advice.

The skeptic's corner

The case against this topic, from the research itself.

Did electric light steal our sleep? Less than the story says. Across half a century of studies, measured adult sleep showed no association with study year, and the US short-sleep rate has held steady recently. Children, whose sleep has shortened slightly year by year, are the exception.69 Pre-industrial sleep was not long or pristine either.234250 What electricity reliably does is make sleep later.

Blue light is oversold. A National Sleep Foundation panel agreed that screen use harms sleep health in children and adolescents, but reached no consensus on adults, or at any age on whether the light itself is the culprit. Blue-filtering interventions showed "only minimal improvements,"70 and a Cochrane review of the trials found very low-certainty evidence for filtering lenses.71 Screens displace sleep instead: among Norwegian university students, each extra hour of screen time in bed was associated with 59% higher odds of insomnia symptoms and less sleep.72

Every light-sensor result above is observational: a week of wrist data, sleeves that cover sensors, and bright nights that come with differences in health, housing and work. Several senior authors of the mortality study disclosed co-founding a circadian lighting company.64 Short and long sleep both predict death, and the cohort that showed it says plainly that causality is unproven.73 Matthew Walker's Why We Sleep carried several of these claims past their evidence, the natural killer cell figure included.68

Terms this topic defines

Related topics

Sources cited above

  1. 1.Foer, J., and Siffre, M. (2008). "Caveman: An Interview with Michel Siffre." Cabinet, Issue 30 (The Underground), Summer 2008. https://www.cabinetmagazine.org/issues/30/foer.php (Quotations verbatim. Siffre gives his sleep/wake cycle as about twenty-four hours and thirty minutes.)
  2. 2.Institut national de l'audiovisuel (n.d.). "1962 : Michel Siffre vit 61 jours au fond d'un gouffre." https://www.ina.fr/ina-eclaire-actu/1962-michel-siffre-vit-61-jours-au-fond-d-un-gouffre ; Institut national de l'audiovisuel (n.d.). "17 septembre 1962 : la remontée de Michel Siffre." https://www.ina.fr/ina-eclaire-actu/video/cpf09003585/17-septembre-1962-la-remontee-de-michel-siffre (INA gives 61 days, about 100 m depth, sub-zero temperature and 98% humidity, and dates the ascent to 17 September; Siffre himself gave 14 September. Depth is given as 130 m in his obituary: Boston Globe, 4 September 2024, https://www.bostonglobe.com/2024/09/04/world/michel-siffre-scientist-solitude-deep-within-caves-dies-85/ .)
  3. 3.Hussey, K.D. (2023). "Timeless spaces: Field experiments in the physiological study of circadian rhythms, 1938-1963." History and Philosophy of the Life Sciences, 45(2), 17. https://doi.org/10.1007/s40656-023-00571-w (By this reading Siffre lived about 42 physiological days against about 58 above ground.)
  4. 69.Youngstedt, S.D., Goff, E.E., Reynolds, A.M., Kripke, D.F., et al. (2016). "Has adult sleep duration declined over the last 50+ years?" Sleep Medicine Reviews, 28, 69-85. https://doi.org/10.1016/j.smrv.2015.08.004 ; Bin, Y.S., Marshall, N.S., and Glozier, N. (2012). "Secular trends in adult sleep duration: a systematic review." Sleep Medicine Reviews, 16(3), 223-230. https://doi.org/10.1016/j.smrv.2011.07.003 ; Matricciani, L., Olds, T., and Petkov, J. (2012). "In search of lost sleep: secular trends in the sleep time of school-aged children and adolescents." Sleep Medicine Reviews, 16(3), 203-211. https://doi.org/10.1016/j.smrv.2011.03.005 ; US Centers for Disease Control and Prevention (2024). Adults: Sleep Facts and Stats. https://www.cdc.gov/sleep/data-research/facts-stats/adults-sleep-facts-and-stats.html (168 studies of 6,052 adults, 1960 to 2013; US short-sleep rate steady from 2013 to 2022; children's sleep fell by a median 0.75 minutes per night per year over the last century.)
  5. 23.Yetish, G., Kaplan, H., Gurven, M., Wood, B., et al. (2015). "Natural sleep and its seasonal variations in three pre-industrial societies." Current Biology, 25(21), 2862-2868. https://doi.org/10.1016/j.cub.2015.09.046 (Hadza, Ju/'hoan San and Tsimane; Actiwatch-2 recordings; night-time light remained below 5.0 lux, the sensor's lower limit, despite small fires. The first edition claimed year-round sameness in human sleep; these groups show seasonal variation instead.) (Sleep onset averaged 3.3 hours after sunset; winter sleep ran longer by 56 minutes among the Tsimane and 53 among the San.)
  6. 42.Samson, D.R., Crittenden, A.N., Mabulla, I.A., Mabulla, A.Z., and Nunn, C.L. (2017). "Hadza sleep biology: Evidence for flexible sleep-wake patterns in hunter-gatherers." American Journal of Physical Anthropology, 162(3), 573-582. https://doi.org/10.1002/ajpa.23160 (33 participants, 393 days of actigraphy; naps averaged 47.5 minutes.) (6.25 hours of sleep at 68.9% efficiency; naps on 54% of days.)
  7. 50.Samson, D.R., Manus, M.B., Krystal, A.D., Fakir, E., et al. (2017). "Segmented sleep in a nonelectric, small-scale agricultural society in Madagascar." American Journal of Human Biology, 29(4), e22979. https://doi.org/10.1002/ajhb.22979 (n=21; 292 days of actigraphy; 6.5 hours of sleep; naps on 89% of days.) (Waking after midnight on 49% of nights.)
  8. 70.Hartstein, L.E., Mathew, G.M., Reichenberger, D.A., Rodriguez, I., et al. (2024). "The impact of screen use on sleep health across the lifespan: A National Sleep Foundation consensus statement." Sleep Health, 10(4), 373-384. https://doi.org/10.1016/j.sleh.2024.05.001
  9. 71.Singh, S., Keller, P.R., Busija, L., McMillan, P., et al. (2023). "Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults." Cochrane Database of Systematic Reviews, 8(8), CD013244. https://doi.org/10.1002/14651858.CD013244.pub2 (17 trials.)
  10. 72.Hjetland, G.J., Skogen, J.C., Hysing, M., Gradisar, M., and Sivertsen, B. (2025). "How and when screens are used: comparing different screen activities and sleep in Norwegian university students." Frontiers in Psychiatry, 16, 1548273. https://doi.org/10.3389/fpsyt.2025.1548273 (Cross-sectional; one author is employed by sleep-app companies, disclosed.) (45,202 students; each extra hour of screen time in bed went with 24 minutes less sleep.)
  11. 64.Windred, D.P., Burns, A.C., Lane, J.M., Olivier, P., et al. (2024). "Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals." PNAS, 121(43), e2405924121. https://doi.org/10.1073/pnas.2405924121 (One week of wrist light sensing; 3,750 deaths over about 8 years; senior authors co-founded a circadian lighting company, disclosed.) (n=88,905; brightest daytime decile against darkest half, hazard ratios 0.66 to 0.83; brightest nights 1.21 to 1.34.)
  12. 73.Cappuccio, F.P., D'Elia, L., Strazzullo, P., and Miller, M.A. (2010). "Sleep duration and all-cause mortality: a systematic review and meta-analysis of prospective studies." Sleep, 33(5), 585-592. https://doi.org/10.1093/sleep/33.5.585 (27 cohorts, 1,382,999 people: short sleep relative risk 1.12, long sleep 1.30.) ; Kripke, D.F., Garfinkel, L., Wingard, D.L., Klauber, M.R., and Marler, M.R. (2002). "Mortality associated with sleep duration and insomnia." Archives of General Psychiatry, 59(2), 131-136. https://doi.org/10.1001/archpsyc.59.2.131
  13. 68.Guzey, A. (2019, updated 2025). "Matthew Walker's 'Why We Sleep' is riddled with scientific and factual errors." https://guzey.com/books/why-we-sleep/ (A critical essay whose individual checks are verifiable; the book's general message that sleep matters is not in dispute.)
  14. 38.Brown, T.M., Brainard, G.C., Cajochen, C., Czeisler, C.A., et al. (2022). "Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults." PLOS Biology, 20(3), e3001571. https://doi.org/10.1371/journal.pbio.3001571 (Targets are melanopic equivalent daylight illuminance measured at the eye; several authors disclose lighting-industry ties.)
  15. 75.Khalsa, S.B., Jewett, M.E., Cajochen, C., and Czeisler, C.A. (2003). "A phase response curve to single bright light pulses in human subjects." Journal of Physiology, 549(Pt 3), 945-952. https://doi.org/10.1113/jphysiol.2003.040477
  16. 37.Burns, A.C., Saxena, R., Vetter, C., Phillips, A.J.K., et al. (2021). "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, 295, 347-352. https://doi.org/10.1016/j.jad.2021.08.056 (Participants reported a median 2.5 hours a day outdoors.)
  17. 66.Mason, I.C., Grimaldi, D., Reid, K.J., Warlick, C.D., et al. (2022). "Light exposure during sleep impairs cardiometabolic function." PNAS, 119(12), e2113290119. https://doi.org/10.1073/pnas.2113290119 (Small laboratory study; exact sample size not stated in the abstract.) (Sleeping under 100 lux against under 3 lux.)
  18. 29.Phillips, A.J.K., Vidafar, P., Burns, A.C., McGlashan, E.M., et al. (2019). "High sensitivity and interindividual variability in the response of the human circadian system to evening light." PNAS, 116(24), 12019-12024. https://doi.org/10.1073/pnas.1901824116 (n=55; evening exposures of 10, 30 and 50 lux delayed melatonin onset by 22, 77 and 109 minutes; individual thresholds ranged from 6 lux to 350.)
  19. 63.Windred, D.P., Burns, A.C., Lane, J.M., Saxena, R., et al. (2024). "Sleep regularity is a stronger predictor of mortality risk than sleep duration: A prospective cohort study." Sleep, 47(1), zsad253. https://doi.org/10.1093/sleep/zsad253 (60,977 participants, mean age 62.8, over 10 million hours of accelerometry.) (The four most regular sleep quintiles had 20% to 48% lower all-cause mortality than the least regular.)
  20. 76.Watson, N.F., Badr, M.S., Belenky, G., Bliwise, D.L., et al. (2015). "Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society." Sleep, 38(6), 843-844. https://doi.org/10.5665/sleep.4716
  21. 41.Stothard, E.R., McHill, A.W., Depner, C.M., Birks, B.R., et al. (2017). "Circadian entrainment to the natural light-dark cycle across seasons and the weekend." Current Biology, 27(4), 508-513. https://doi.org/10.1016/j.cub.2016.12.041 (Winter study n=5; weekend study n=9 campers vs 5 at home.) (Winter daylight more than 13 times brighter than indoor December life; melatonin onset 2.6 hours earlier. The weekend produced about 69% of the week's shift.)
  22. 77.Schöllhorn, I., Stefani, O., Lucas, R.J., Spitschan, M., et al. (2023). "Melanopic irradiance defines the impact of evening display light on sleep latency, melatonin and alertness." Communications Biology, 6(1), 228. https://doi.org/10.1038/s42003-023-04598-4 (72 healthy men; some authors hold display patents, disclosed.)
  23. 78.Drake, C., Roehrs, T., Shambroom, J., and Roth, T. (2013). "Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed." Journal of Clinical Sleep Medicine, 9(11), 1195-1200. https://doi.org/10.5664/jcsm.3170 (n=12, randomized, double-blind.) ; Burke, T.M., Markwald, R.R., McHill, A.W., Chinoy, E.D., et al. (2015). "Effects of caffeine on the human circadian clock in vivo and in vitro." Science Translational Medicine, 7(305), 305ra146. https://doi.org/10.1126/scitranslmed.aac5125
  24. 79.Qaseem, A., Kansagara, D., Forciea, M.A., Cooke, M., and Denberg, T.D. (2016). "Management of chronic insomnia disorder in adults: a clinical practice guideline from the American College of Physicians." Annals of Internal Medicine, 165(2), 125-133. https://doi.org/10.7326/M15-2175 ; Trauer, J.M., Qian, M.Y., Doyle, J.S., Rajaratnam, S.M., and Cunnington, D. (2015). "Cognitive behavioral therapy for chronic insomnia: a systematic review and meta-analysis." Annals of Internal Medicine, 163(3), 191-204. https://doi.org/10.7326/M14-2841
  25. 80.Sateia, M.J., Buysse, D.J., Krystal, A.D., Neubauer, D.N., and Heald, J.L. (2017). "Clinical practice guideline for the pharmacologic treatment of chronic insomnia in adults: an American Academy of Sleep Medicine clinical practice guideline." Journal of Clinical Sleep Medicine, 13(2), 307-349. https://doi.org/10.5664/jcsm.6470 ; Cohen, P.A., Avula, B., Wang, Y.H., Katragunta, K., and Khan, I. (2023). "Quantity of melatonin and CBD in melatonin gummies sold in the US." JAMA, 329(16), 1401-1402. https://doi.org/10.1001/jama.2023.2296 (22 of 25 products inaccurately labeled.)

This topic carries 80 notes in the research manuscript; the 25 above are those behind the material on this page. Full source register.