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Chapter 7 of the book

Light and Dark

The next four chapters follow the signals that place a body in time and space: light and dark, the sleep that light schedules, heat and cold, green space and quiet. Blur one of those signals and the system built to read it drifts.

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.3 things to start with3 popular claims graded87 works cited

How the chapter opens

In July 1962 a 23-year-old French geologist named Michel Siffre climbed down into a cave in the Alps to live beside an underground glacier. His camp sat more than a hundred meters down, on the ice, below freezing, in air that was almost saturated with water. "My feet were always wet," he said later.

He took no watch and no calendar, and no daylight reached him. 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." Time came loose. Counting to 120, which should take two minutes, took him five. He had planned to come up in mid-September. When the surface team told him the day had arrived, he was astonished. "I thought that it was only August 20. I believed I still had another month to spend in the cave." He came up exhausted, his eyes shielded by opaque glasses against the daylight.1

What the chapter covers

The argument, the studies and the stories are in the book. Get the book.

  • ·What Your Body Expects
  • ·What It Gets Instead
  • ·The Indoor Eye
  • ·Sun on the Wrong Skin

Mismatch card

The system

The circadian clock, set through melanopsin in the eye; skin pigment and vitamin D; the growing eye.

What it expects

Bright days, truly dark nights, sun matched to skin and changing with the seasons, and a childhood spent largely outdoors.

What it gets

Days mostly indoors under room light far dimmer than any sky, evenings lit brightly enough to hold the clock back, lit bedrooms, pale skin under sudden holiday sun and dark skin in indoor lives, and classrooms instead of fields.

What happens in the gap

The clock drifts later, sleep timing scatters, vitamin D runs low in some skins while sunburn and skin cancer rise in others, and children's eyes grow too long.

The handrail

Morning daylight outdoors, dim evenings, a dark bedroom, unburnt sun matched to skin type, vitamin D where sun falls short, and two hours outside for children (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.)

Strength of evidence

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. Light's effect on the clock and outdoor time's effect on myopia rest on trials; the links from light to disease and death rest on observational cohorts, and vitamin D pills have not prevented major disease in adults who had enough.

What you've heard

Popular claims this chapter tests, each with the book's verdict and grade.

The protocol

Each action carries the book's evidence grade. How the grades work.

Start here

  1. Get outdoor daylight early in your day, roughly 10 to 30 minutes

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

    How: within an hour of waking, step outside, not behind glass: walk part of the commute, coffee on the step, the dog. Longer when gray or shaded; open sky beats a courtyard; a lunchtime walk outside counts too. Sunglasses are fine. Why: morning light pulls the clock earlier, and outdoors is many times brighter than indoors.

  2. Dim the last two to three hours before bed

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

    How: switch off overhead lights and use one or two low lamps, warm white if you are buying bulbs; turn screen brightness down. Many ordinary warm-white bulbs, kept low, already meet the level sleep researchers suggest for the evening. Why: an ordinary living room is bright enough to delay melatonin, and people differ so much in sensitivity that it is better to judge by your own sleep than to chase a number.20

  3. Get children outdoors about two hours a day; less still helps

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

    How: recess, walking to school, afternoons in the park, weekends outside. Shade and gray skies count; indoor sport did not show the same link. It matters most from school entry on, and in one trial a forty-minute outdoor class each school day helped. Why: outdoor time roughly halves new cases in trials, and Taiwan set two hours as its school target. It prevents new cases more than it slows existing ones (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. for slowing).21

Then, pick from this menu

  • **Make the bedroom properly dark (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for sleep physiology; EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. for long-term health).** Curtains that work, standby lights covered, the phone face down or outside the room: dark enough that you could not read. If you are over 60 or get up at night, use a dim, low night light on the route to the bathroom rather than risk a fall. Sleep itself is Chapter 8.

  • **Match your sun to your skin (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** Short, regular, unburnt sun; shade, clothing and a hat for anything longer; then sunscreen. When the UV index is 3 or above, Cancer Council Australia advises SPF 50 or 50+, broad-spectrum and water resistant, as "the last line of defence." Use it to prevent burning, not to stay out longer: in a blinded trial, young Europeans given a higher SPF spent more time in the sun and burned just as often. Skip sunbeds; first use before 35 is linked to 87% higher melanoma risk.22

  • **Take vitamin D when the sun cannot supply it (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. against deficiency; EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. against cancer, heart disease or depression).** Britain's National Health Service suggests 10 micrograms (400 IU) a day in autumn and winter, all year with dark skin or little time outdoors, and no more than 100 micrograms (4,000 IU) a day over age ten unless a doctor advises otherwise. Prefer daily doses to large monthly ones. Test only with a reason, such as osteoporosis, malabsorption or kidney disease; an expert panel suggests against routine screening, even for people with dark skin.23

  • **Get children's eyes checked, and if myopia is getting worse, ask about myopia control (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** Vision screening at least once between ages 3 and 5, an eye examination before first grade, and sooner if a child squints at distance. In 2025 the US Food and Drug Administration authorized the first spectacle lenses designed to slow myopia for children from six to twelve, and special contact lenses and eye drops are other options. Atropine drops can bring light sensitivity and blurred near vision, and lenses worn overnight carry a small risk of eye infection. Ask an eye doctor which suits your child.24

  • **Check your own skin, and know the ABCDEs (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.). The American Academy of Dermatology's warning signs for melanoma: Asymmetry, "one half of the spot is unlike the other half"; Border, "irregular, scalloped, or poorly defined"; Color, "varying colors from one area to the next"; Diameter, usually about the size of a pencil eraser or larger when found, though melanomas "can be smaller"; E**volving, a spot that "looks different from the rest or is changing in size, shape, or color." Show a changing spot to a clinician. No trial shows that self-checks save lives, and the US Preventive Services Task Force found the evidence insufficient to judge routine skin checks by a clinician (Chapter 16).25

Your skinSun and cancerSun and vitamin DWhat to do
Fair: burns easily, tans littleHighest melanoma risk; burns are the main dangerAbout thirteen minutes of midday summer sun, a third of the skin bare, three times a week, lifted nine in ten white adults in one English study out of deficiencyShort sun on most days, never to pink; check the day's UV index, and at 3 or above cover up for anything longer
Medium: tans readily, sometimes burnsLower risk than fair skin, but not zeroSomewhat longer than fair skinStop well before any pink; same cover-up rules for long exposure
Dark brown to black: rarely burnsMelanoma is rare, and tends to appear on palms, soles and other skin the sun rarely reachesMelanin slows production; on the same routine, South Asian adults all stayed lowConsider a vitamin D supplement all year; check palms and soles

If this is you

Night shifts

reverse the timing. Bright light during the shift if you can get it, the darkest sunglasses you can safely wear outdoors on the way home (never lenses so dark they impair driving), and the same dark daytime sleep slot every workday. The package moved the clock toward nights in lab studies, but evidence from real workplaces is thin, so treat it as worth trying (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.). If you are sleepy, nap before you drive.26

One window and a desk job

the protocol means outdoors, so use the door: first break, a phone call or lunch outside, even in shade.

Short on time

stack it on what you already do, like the school run on foot or coffee on the step. Walking your child to school counts toward the first and third "Start here" actions at once.

Bad knee, or can't walk far

the dose is the light, not the walk. Sitting on a bench or a step under open sky counts, and even tree shade gives many times the light of a room.

Darker skin

take vitamin D seriously in winter or if you are mostly indoors, and look at your palms and soles when you check your skin.

Fair skin, many moles, past skin cancer, medicines that make skin sensitive to light, or an organ transplant

follow your own clinician's advice on sun.

Parents of babies

keep babies under six months in shade and clothing, and ask a pediatrician about sunscreen.

Tight budget

everything in "Start here" is free. The one purchase worth considering is a warm-white bulb for the lamp you use after dark.

Safety

Never look at the sun. With strong nearsightedness, a burst of new floaters, flashes of light, or a dark curtain across your vision means a same-day visit to an eye doctor or emergency room. With kidney disease, sarcoidosis or high blood calcium, ask a doctor before taking vitamin D. Red-light devices sold to slow myopia can exceed laser safety limits and belong in supervised trials only.27 None of this replaces an eye examination or treatment for a sleep disorder.

The skeptic's box

What could overturn this chapter, from the chapter itself.

The light-and-mortality results are observational. A week of wrist data stands in for years, sleeves cover sensors, and bright nights come with differences in health, housing and work. The two senior authors of the largest study co-founded a circadian-health company and have taken funding from lighting manufacturers. That does not make the sensors lie, but it is a reason to wait for other cohorts. The camping studies are tiny, and nobody has tested the morning hour or the ten to thirty minutes. The case for outdoor time in children is stronger, yet its effects are modest: in a Guangzhou trial, a daily outdoor class left a gap of nine percentage points over three years, and the light mechanism is proven only in animals. Taiwan's national fall came from an eye chart, and careful measurements in Taipei show second-grade myopia still creeping up, more slowly.28 Genes set who is susceptible to myopia; environment sets how many. Vitamin D has the opposite problem, with strong associations and weak trials, because most trial volunteers already had enough. A large trial in people who are truly deficient would change the picture.

Go deeper in the reference library

The research this chapter was built from, as longer reference entries: each with its own mismatch card, graded recommendations and the case against it.

See also in the book

Sources behind this page

  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.) ; 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 (About 100 m down, sub-zero temperature, 98% humidity.) ; 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
  2. 28.He et al. (2015), note 21 (30.4% against 39.5%). ; Yang, Y.C., Fang, S.Y., Yang, H.Y., Wang, C.Y., et al. (2026). "Evolving patterns of childhood myopia in Taiwan: Effects of urbanization, digitalization, and outdoor activity policies." Taiwan Journal of Ophthalmology, 16(2), 294-302. https://doi.org/10.4103/tjo.TJO-D-25-00213 (Measured myopia in Taipei second graders was 36.36% in 2013 and 39.39% in 2023.)
  3. 9.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 (Seventeen trials.) ; Shechter, A., Quispe, K.A., Mizhquiri Barbecho, J.S., Slater, C., et al. (2020). "Interventions to reduce short-wavelength ('blue') light exposure at night and their effects on sleep: a systematic review and meta-analysis." Sleep Advances, 1(1), zpaa002. https://doi.org/10.1093/sleepadvances/zpaa002 ; 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
  4. 11.Chang, A.M., Santhi, N., St Hilaire, M., Gronfier, C., et al. (2012). "Human responses to bright light of different durations." Journal of Physiology, 590(13), 3103-3112. https://doi.org/10.1113/jphysiol.2011.226555 (Evening light after a dim-light baseline; 12 minutes gave a 1.07-hour delay against 2.65 hours for 4 hours.) ; Dewan, K., Benloucif, S., Reid, K., Wolfe, L.F., et al. (2011). "Light-induced changes of the circadian clock of humans: increasing duration is more effective than increasing light intensity." Sleep, 34(5), 593-599. https://doi.org/10.1093/sleep/34.5.593
  5. 19.Matta, M.K., Zusterzeel, R., Pilli, N.R., Patel, V., et al. (2019). "Effect of sunscreen application under maximal use conditions on plasma concentration of sunscreen active ingredients: a randomized clinical trial." JAMA, 321(21), 2082-2091. https://doi.org/10.1001/jama.2019.5586 ; U.S. Food and Drug Administration (2026). Questions and answers: FDA's regulatory actions on over-the-counter sunscreen. https://www.fda.gov/drugs/understanding-over-counter-medicines/questions-and-answers-fdas-regulatory-actions-over-counter-sunscreen ; Green, A., Williams, G., Neale, R., Hart, V., et al. (1999). "Daily sunscreen application and betacarotene supplementation in prevention of basal-cell and squamous-cell carcinomas of the skin: a randomised controlled trial." The Lancet, 354(9180), 723-729. https://doi.org/10.1016/S0140-6736(98)12168-2 (1,621 adults. Tumors, rate ratio 0.61; people with a new squamous cell cancer, 0.88, 95% CI 0.50-1.56.) ; Green, A.C., Williams, G.M., Logan, V., and Strutton, G.M. (2011). "Reduced melanoma after regular sunscreen use: randomized trial follow-up." Journal of Clinical Oncology, 29(3), 257-263. https://doi.org/10.1200/JCO.2010.28.7078 (Invasive melanoma hazard ratio 0.27, 95% CI 0.08-0.97; all melanomas 0.50, 0.24-1.02.)
  6. 20.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 (Evening and night targets can be met by common warm-white LEDs at about 30 lux; several authors disclose lighting-industry ties.) For individual differences, see Phillips et al. (2019), note 8.
  7. 21.Rose, K.A., Morgan, I.G., Ip, J., Kifley, A., et al. (2008). "Outdoor activity reduces the prevalence of myopia in children." Ophthalmology, 115(8), 1279-1285. https://doi.org/10.1016/j.ophtha.2007.12.019 (Indoor sport showed no association.) ; He, M., Xiang, F., Zeng, Y., Mai, J., et al. (2015). "Effect of time spent outdoors at school on the development of myopia among children in China: A randomized clinical trial." JAMA, 314(11), 1142-1148. https://doi.org/10.1001/jama.2015.10803 (1,903 first graders; a forty-minute outdoor class each school day for three years; 30.4% against 39.5%.) Pooled trials and Taiwan's target: note 15.
  8. 26.Crowley, S.J., Lee, C., Tseng, C.Y., Fogg, L.F., and Eastman, C.I. (2003). "Combinations of bright light, scheduled dark, sunglasses, and melatonin to facilitate circadian entrainment to night shift work." Journal of Biological Rhythms, 18(6), 513-523. https://doi.org/10.1177/0748730403258422 ; Smith, M.R., Fogg, L.F., and Eastman, C.I. (2009). "A compromise circadian phase position for permanent night work improves mood, fatigue, and performance." Sleep, 32(11), 1481-1489. https://doi.org/10.1093/sleep/32.11.1481 (Volunteers whose clocks shifted had near-daytime mood and performance.) ; Slanger, T.E., Gross, J.V., Pinger, A., Morfeld, P., et al. (2016). "Person-directed, non-pharmacological interventions for sleepiness at work and sleep disturbances caused by shift work." Cochrane Database of Systematic Reviews, 2016(8), CD010641. https://doi.org/10.1002/14651858.CD010641.pub2 (The one small field trial of light plus dawn sunglasses found no effect on sleepiness.) ; Wright, K., Abbott, S., Ashbrook, L., Drake, C., et al. (2026). "0778 Management of Shift Work Disorder: an American Academy of Sleep Medicine clinical practice guideline." Sleep, 49(Supplement 1), A347-A349. https://doi.org/10.1093/sleep/zsag091.0777 (Conditional suggestions, nearly all on low or very low certainty evidence.)
  9. 27.National Eye Institute. Retinal detachment. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinal-detachment ; Ostrin, L.A., and Schill, A.W. (2026). "Safety evaluation of 4 red light therapy devices for myopia." JAMA Ophthalmology, 144(3), 255-258. https://doi.org/10.1001/jamaophthalmol.2025.5660
  10. 22.Cancer Council Australia (n.d.). Vitamin D. https://www.cancer.org.au/cancer-information/causes-and-prevention/sun-safety/vitamin-d ; Cancer Council Australia (n.d.). About sunscreen. https://www.cancer.org.au/cancer-information/causes-and-prevention/sun-safety/about-sunscreen ; Autier, P., Doré, J.F., Négrier, S., Liénard, D., et al. (1999). "Sunscreen use and duration of sun exposure: a double-blind, randomized trial." Journal of the National Cancer Institute, 91(15), 1304-1309. https://doi.org/10.1093/jnci/91.15.1304 (SPF 30 against SPF 10.) ; Clemens, T.L., Adams, J.S., Henderson, S.L., and Holick, M.F. (1982). "Increased skin pigment reduces the capacity of skin to synthesise vitamin D3." The Lancet, 319(8263), 74-76. https://doi.org/10.1016/s0140-6736(82)90214-8 ; Khlat, M., Vail, A., Parkin, M., and Green, A. (1992). "Mortality from melanoma in migrants to Australia: variation by age at arrival and duration of stay." American Journal of Epidemiology, 135(10), 1103-1113. https://doi.org/10.1093/oxfordjournals.aje.a116210 (Migrants from Southern Europe and East Asia were the least likely to die of melanoma.) ; Boniol, M., Autier, P., Boyle, P., and Gandini, S. (2012). "Cutaneous melanoma attributable to sunbed use: systematic review and meta-analysis." BMJ, 345, e4757. https://doi.org/10.1136/bmj.e4757 . The table's vitamin D doses are from Rhodes et al. (2010) and Farrar et al. (2011), note 16.
  11. 23.NHS (2020). Vitamin D (page last reviewed 3 August 2020). https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ ; Demay, M.B., Pittas, A.G., Bikle, D.D., Diab, D.L., et al. (2024). "Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline." Journal of Clinical Endocrinology and Metabolism, 109(8), 1907-1947. https://doi.org/10.1210/clinem/dgae290 (For people over 50 who take vitamin D, suggests daily rather than intermittent high doses.)
  12. 24.US Preventive Services Task Force; Grossman, D.C., Curry, S.J., Owens, D.K., et al. (2017). "Vision screening in children aged 6 months to 5 years: US Preventive Services Task Force recommendation statement." JAMA, 318(9), 836-844. https://doi.org/10.1001/jama.2017.11260 ; US Food and Drug Administration (2025). FDA authorizes marketing of first eyeglass lenses to slow progression of pediatric myopia. https://www.fda.gov/news-events/press-announcements/fda-authorizes-marketing-first-eyeglass-lenses-slow-progression-pediatric-myopia ; Lawrenson, J.G., Huntjens, B., Virgili, G., Ng, S., et al. (2025). "Interventions for myopia control in children: a living systematic review and network meta-analysis." Cochrane Database of Systematic Reviews, 2(2), CD014758. https://doi.org/10.1002/14651858.CD014758.pub3 ; Chua, W.H., Balakrishnan, V., Chan, Y.H., Tong, L., et al. (2006). "Atropine for the treatment of childhood myopia." Ophthalmology, 113(12), 2285-2291. https://doi.org/10.1016/j.ophtha.2006.05.062 ; Bullimore, M.A., Sinnott, L.T., and Jones-Jordan, L.A. (2013). "The risk of microbial keratitis with overnight corneal reshaping lenses." Optometry and Vision Science, 90(9), 937-944. https://doi.org/10.1097/OPX.0b013e31829cac92 (About 14 cases per 10,000 patient-years in US children.)
  13. 25.American Academy of Dermatology (n.d.). What to look for: ABCDEs of melanoma. https://www.aad.org/public/diseases/skin-cancer/find/at-risk/abcdes (Read September 2026.) ; US Preventive Services Task Force; Mangione, C.M., Barry, M.J., Nicholson, W.K., et al. (2023). "Screening for skin cancer: US Preventive Services Task Force recommendation statement." JAMA, 329(15), 1290-1295. https://doi.org/10.1001/jama.2023.4342 (An I statement: evidence insufficient, not evidence of no benefit.)

Every work this chapter cites

The chapter's 28 notes cite 87 works, listed by note number. Each is also in the full source register.

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  5. 5.Pacchioli, D (2015). The sepia rainbow. Penn State News. psu.eduBranda, R.F., and Eaton, J.W (1978). Skin color and nutrient photolysis: an evolutionary hypothesis. Science. 10.1126/science.675247Jablonski, N.G., and Chaplin, G (2000). The evolution of human skin coloration. Journal of Human Evolution. 10.1006/jhev.2000.0403Perretti, S., Santos, P., Vizzari, M.T., Tassani, E., et al (2025). Inference of human pigmentation from ancient DNA by genotype likelihoods. PNAS. 10.1073/pnas.2502158122
  6. 6.Ashby, R., Ohlendorf, A., and Schaeffel, F (2009). The effect of ambient illuminance on the development of deprivation myopia in chicks. Investigative Ophthalmology & Visual Science. 10.1167/iovs.09-3419Ashby, R., Harb, E.N., Ostrin, L.A., Flitcroft, D.I., et al (2025). IMI: The role of light in refractive development and myopia: Evidence from animal and human studies. Investigative Ophthalmology & Visual Science. 10.1167/iovs.66.15.5
  7. 7.Klepeis, N.E., Nelson, W.C., Ott, W.R., Robinson, J.P., et al (2001). The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants. Journal of Exposure Analysis and Environmental Epidemiology. 10.1038/sj.jea.7500165Scheuermaier, K., Laffan, A.M., and Duffy, J.F (2010). Light exposure patterns in healthy older and young adults. Journal of Biological Rhythms. 10.1177/0748730410361916Espiritu, R.C., Kripke, D.F., Ancoli-Israel, S., Mowen, M.A., et al (1994). Low illumination experienced by San Diego adults: association with atypical depressive symptoms. Biological Psychiatry. 10.1016/0006-3223(94Lanca, C., Teo, A., Vivagandan, A., Htoon, H.M., et al (2019). The effects of different outdoor environments, sunglasses and hats on light levels: implications for myopia prevention. Translational Vision Science & Technology. 10.1167/tvst.8.4.7Bhandary, S.K., Dhakal, R., Sanghavi, V., and Verkicharla, P.K (2021). Ambient light level varies with different locations and environmental conditions: Potential to impact myopia. PLOS ONE. 10.1371/journal.pone.0254027
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