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Chapter 8 of the book

Sleep

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.3 things to start with4 popular claims graded85 works cited

How the chapter opens

Researchers once asked people in three foraging societies with no electric light to wear a small monitor on the wrist through the night: the Hadza of Tanzania, the San of Namibia and the Tsimane of Bolivia. The monitors tracked movement, and so sleep, as well as the light reaching each wrist, while loggers recorded the temperature of the air.1

If the popular story about ancestral sleep were true, these people would have lain down at dusk and slept long hours in the dark. None of the three groups did. Sleep began, on average, more than three hours after sunset. Small fires burned in the camps, yet the light at the wrist stayed too dim for the sensors to register. People usually woke before sunrise, after between about five and three-quarter and seven hours, near the low end of what people in industrial countries get.

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
  • ·Insomnia, and the Treatment That Works
  • ·The Night Shift
  • ·When Snoring Is a Symptom

Mismatch card

The system

The circadian clock and the sleep it schedules, tuned by light, temperature and routine.

What it expects

A bright day, a dark night that cools, and the same timing tomorrow; enough sleep, with room for naps.

What it gets

Dim days, lit evenings, a bedroom at one temperature with a phone in it, a schedule that lurches at weekends, afternoon caffeine, and for some, work through the night.

What happens in the gap

Sleep moves later and grows irregular; insomnia is common and often untreated; night workers live against their own clocks; apnea goes undiagnosed, especially in women after menopause.

The handrail

A fixed wake time, a dark and cooling bedroom without a phone, CBT-I for chronic insomnia, and snoring taken seriously. (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. CBT-I and the apnea treatments rest on trials; the regularity link rests on a large but observational cohort; the shift-work fixes on laboratory simulations.

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 up at the same time every day, weekends included

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

    How: pick a wake time you can keep on your busiest day, and hold it on Saturday too. After a bad night, get up anyway. Why: regular timing predicted survival better than duration did in the largest measured study, though it is observational.

  2. Make the bedroom dark, cool and quiet, with the phone charging somewhere else

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.for dark, for sleep timingEmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.for cool, for the phone in adults, and for heart and metabolic effects

    How: blackout curtains or a blind that works; tape or cover standby lights; let the room cool through the night and adjust the bedding instead of chasing a number on the thermostat; earplugs if noise wakes you. Put a cheap alarm clock by the bed and the phone across the room or outside it. Why: light late in the day pushes the body clock later, and screens in bed go with less sleep.

  3. If insomnia has lasted three months or more, ask for CBT-I before sleeping pills

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

    How: ask your doctor for it by name. It works by telephone; in one trial it took six calls over eight weeks. Courses and apps also exist, so ask which ones your doctor trusts. Why: it is the recommended first treatment for every adult with chronic insomnia. Do not stop a prescribed sleep medicine abruptly without medical advice.

Then, pick from this menu

  • **Aim for seven hours or more of sleep opportunity (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** Most adults need at least that, and short sleep builds deficits you will not feel.

  • **Take snoring seriously (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.).** Loud snoring, observed pauses or gasping, or sleepiness despite enough time in bed: see a doctor about a sleep study. Losing weight, if you carry extra, reduces apnea.

  • **Stop caffeine at least six hours before bed (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. that afternoon caffeine can cost sleep; EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. for the exact cutoff).** If you sleep at eleven, your last coffee, tea, cola or energy drink is by five. Allow longer if you take oral contraceptives. If you need an afternoon lift, make it earlier or smaller, and if you still lie awake, move the cutoff an hour earlier and see. Why: the body clears only half a dose every three to seven hours, so much of an afternoon cup is still active at bedtime.

  • **Get outdoor daylight early in your day and dim the last two to three hours before bed (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** Roughly 10 to 30 minutes outdoors, not through a window, longer under cloud; then fewer, lower lamps in the evening. Chapter 7 gives the doses.

  • **Take a warm bath or shower one to two hours before bed (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** Water that feels hot but comfortable, for as little as ten minutes, shortened the time to fall asleep across pooled studies.25

  • **Nap if you want to (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.).** Some foragers nap on most days. Waking from deep sleep brings grogginess, which, without serious sleep loss, rarely lasts beyond half an hour, so give it that long before you drive.26 If you have insomnia, ask whoever treats it whether to nap.

  • **Treat a sleep tracker as a rough diary (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** Consumer trackers are good at telling sleep from wake in bed but count lying awake as sleep, and their "deep" and "REM" numbers are unreliable. Use one for timing and regularity, if at all; if its scores make you anxious about sleep, stop wearing it. (Chapter 1 covers chasing the score.)27

  • **Use melatonin for jet lag, not insomnia (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** For flights across five or more time zones, a dose of 0.5 to 5 mg taken close to bedtime at the destination reduced jet lag in most trials; more than 5 mg was no better. Take it at local bedtime, not earlier in the day, when it can cause sleepiness and slow adjustment, and do not drive after a dose. Ask a clinician first if you have epilepsy or take warfarin.28

If this is you

Night shiftsEmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

nap before the first night shift; caffeine in the first half of the shift only; a snack, not a meal, on shift; bright light at work if you can get it; dark sunglasses outdoors on the way home (if you drive, only as dark as still lets you see the road clearly); the same dark, quiet sleep block after every shift; afternoon daylight on days off; if you have a say in a rotating rota, ask for it to rotate forward, from days to evenings to nights. Dark glasses on the way home are cheap and reasonable, but unproven on their own. If you are sleepy, do not drive until you have napped and the grogginess has passed.

Perimenopause and after

CBT-I works even while hot flushes continue, and treatments for the flushes themselves exist (Chapter 15). New snoring, morning headaches, unrefreshing sleep or new insomnia after menopause deserve a question about apnea (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).

Pregnant

insomnia is common and CBT-I helps; ask for an iron check if restless legs keep you up, and take no sleep medicine or iron supplement without advice (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).

PCOS, diagnosed or suspected

ask about sleep breathing problems, especially if you snore or wake unrefreshed (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).

Phone in bed, or anxious at night

charge the phone outside the room and use a plain alarm clock. If worry keeps you awake most nights for three months or more, that is chronic insomnia, and CBT-I treats it; Chapter 13 covers the anxiety itself (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).

New parents

expect lost sleep for months, not weeks. In a large German study, mothers lost about an hour of sleep a night in the first three months after a birth, and fathers much less; neither had fully recovered years later (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).29 Plan the nights as a team and recruit help (Chapter 11) (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. for any specific plan).

Older, or unsteady at night

keep a dim, low night light on the path to the bathroom; a fall costs more than a little light (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.).

Tight budget

the three changes that matter most are free or close to it. A fixed wake time (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.), a towel over the window or the standby lights (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.), and a phone across the room (EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.).

Short on time

keep only the wake time. It is the one habit that organizes the others (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).

Safety

Do not drive drowsy; after a night shift or a nap, let the grogginess pass first. If you drive home after a night shift, wear the darkest lenses that still let you see the road clearly; very dark glasses are for walking or riding as a passenger. Never stop prescribed sleep medicine abruptly without a clinician. Keep melatonin gummies away from children; their content is unreliable. With a warm bath, especially if you are older, warm the bathroom first, skip alcohol and rise slowly. Untreated sleep apnea raises the risk of crashes, so get it assessed, not taped.

The skeptic's box

What could overturn this chapter, from the chapter itself.

The regularity finding, the chapter's headline, rests on observational cohorts: irregular sleepers differ in work, health and income, and statistics cannot fully remove those differences. The forager data come from small groups studied for weeks. Electric light has not measurably shortened adult sleep over half a century, which cuts against the story that modern life has stolen our hours. The shift-work advice is built from laboratory simulations in young volunteers, and the six-hour caffeine margin from a dozen people given a large dose, so your own cutoff may sit earlier or later. What would change the chapter: a randomized trial in which people assigned to regular sleep timing lived longer or stayed healthier than people who were not. Until then, regularity is the cheapest bet with the best odds, not a proven cure.

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

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  2. 11.Porkka-Heiskanen, T., Strecker, R.E., Thakkar, M., Bjorkum, A.A., et al. (1997). "Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness." Science, 276(5316), 1265-1268. https://doi.org/10.1126/science.276.5316.1265 ; Lovallo, W.R., Whitsett, T.L., al'Absi, M., Sung, B.H., et al. (2005). "Caffeine stimulation of cortisol secretion across the waking hours in relation to caffeine intake levels." Psychosomatic Medicine, 67(5), 734-739. https://doi.org/10.1097/01.psy.0000181270.20036.06 (After five days at 300 or 600 mg a day, a 9 a.m. dose no longer raised cortisol, but a 1 p.m. dose did; the authors conclude that responses are "reduced, but not eliminated" in daily users.) Adenosine data are from the basal forebrain of cats.
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Every work this chapter cites

The chapter's 29 notes cite 85 works, listed by note number. Each is also in the full source register.

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