睡眠

A night runs in cycles

Sleep is not one state but a sequence of cycles, each about ninety minutes long, in which light sleep, deep slow-wave sleep and REM sleep take turns. Deep sleep dominates the first cycles and REM the later ones, so a night cut short at either end loses a different kind of sleep. The ninety-minute figure is an average with wide variation between people and across the night, which is why the cycle planner on this tab labels its windows as estimates rather than promises.

Those stages do real work. Memory for facts and for skills is consolidated during sleep, and people who sleep between two study sessions or two practice sessions relearn faster and retain longer than people who stay awake for the same interval.

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How much is enough

The American Academy of Sleep Medicine and the Sleep Research Society reviewed the evidence together and concluded that adults should sleep seven hours or more a night on a regular basis; the National Sleep Foundation's panel put the healthy range at seven to nine hours. Below seven hours the risks of weight gain, diabetes, high blood pressure, heart disease, depression and accidents rise, and so does all-cause mortality in pooled prospective studies.

Need varies between people, but less than most people assume. When volunteers were given the chance to sleep as long as they wanted for over a week, their nights settled at around eight and a half hours, roughly an hour more than they had been getting, and their daytime alertness improved as the difference was paid back.

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Timing and regularity

Everyone carries an internal clock with its own preferred timing, called a chronotype, and the clock is set mostly by light. Sleep taken at the wrong time for that clock is lighter and shorter, which is the everyday experience of jet lag and of the shift worker.

Regularity is a property of its own. The Sleep Regularity Index scores how likely you are to be in the same state, asleep or awake, at the same time on consecutive days. Students with low scores had a body clock delayed by almost three hours and worse grades than regular sleepers who slept the same total amount, and in a cohort of over sixty thousand adults, regularity predicted mortality more strongly than duration did.

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The patterns people keep

Most adults sleep in one consolidated block, a monophasic pattern. A biphasic pattern adds a regular daytime nap; a polyphasic one splits sleep into several short pieces around the clock. Consolidated sleep is by far the best evidenced, a short daytime nap has good support, and the National Sleep Foundation's expert panel concluded that schedules which chop sleep into many pieces to reduce total sleep are harmful.

This tab does not tell you which pattern to keep. It works out which one you already keep from the nights and naps you log, says how confident it is, and leaves the choice to you. The sections below describe what the research says about each pattern so that the choice is an informed one.

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Attention, memory and mood

Restricting healthy adults to six hours a night for two weeks left their attention and reaction time as impaired as after a full night without sleep, and the deficit kept growing each day. Sleep also consolidates what you learned during the day, from facts to motor skills, so a short night after learning throws part of the effort away. Among university students, the duration, quality and consistency of sleep together explained around a quarter of the variation in grades, and sleepiness is one of the commonest reasons students fall behind.

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Long-term health

Across pooled prospective studies, both short and unusually long sleep are associated with higher mortality. Irregular sleep is linked with higher ten-year cardiovascular risk, obesity, high blood pressure, diabetes and depression in older adults, and the most regular sleepers in a large national cohort had a twenty to forty-eight per cent lower risk of death than the least regular. Each hour of social jetlag, the mismatch between sleep on work days and free days, was associated with higher body weight. Shift work, which forces sleep against the clock, carries its own well-documented costs.

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You cannot feel the deficit

The most useful single finding in the sleep restriction literature is that people adapt to feeling tired long before their performance recovers. After a few days of short nights, the volunteers rated themselves only mildly sleepy while their test scores kept falling. The same is true of caffeine, which shortened objectively measured sleep by more than an hour when taken six hours before bed even though participants did not report a worse night. That is why this tab tracks numbers rather than asking how you feel, and why the debt figure is worth watching.

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Anchor the wake time

The single strongest lever is a wake time that stays within about an hour every day, including days off. The wake time sets the clock through morning light, keeps the regularity index high, and stops the Monday-morning jet lag that follows a late weekend. If you need more sleep, move bedtime earlier rather than the alarm later. Regularity had a stronger relationship with health than duration in the largest cohort, and regular sleepers with the same total sleep as irregular ones had earlier body clocks and better grades.

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The last hours before bed

Caffeine taken even six hours before bed cut total sleep by over an hour in a controlled trial, so the last coffee belongs in the early afternoon. Alcohol brings sleep on faster and then fragments the second half of the night and suppresses REM, which is why a nightcap leaves you unrefreshed. Bright, blue-rich light from a screen in the hour before bed suppressed melatonin, delayed the body clock and reduced next-morning alertness in a controlled crossover study. Evening exercise is fine for most people; only vigorous training that ends within an hour of bed measured worse. The reviews of sleep hygiene rate caffeine and light among the recommendations with the best evidence.

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Move during the day

Regular exercise improves how fast people fall asleep, how long they sleep and how they rate the night, with the biggest effects in those who sleep worst. A single session helps a little on the same night. The activity guidelines that protect the heart, one hundred and fifty minutes of moderate activity a week plus two strength sessions, are the same ones that help sleep, and the evening is not off limits unless the session is vigorous and ends right before bed.

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When sleep will not come

If you lie awake most nights for more than a few weeks, the treatment with the best evidence is cognitive behavioural therapy for insomnia, which trims the time it takes to fall asleep by around twenty minutes and time awake in the night by around half an hour, with effects that hold after treatment ends. Its core moves are usable on your own: keep the bed for sleep, get up if you have been awake for a long stretch, keep the wake time fixed, and do not extend time in bed to chase sleep. Mindfulness practice also improves sleep quality compared with doing nothing. Persistent problems belong with a clinician.

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Catching up at the weekend

Sleeping in on free days feels like repayment, but it moves the clock. The gap between mid-sleep on work days and free days is called social jetlag, and it tracks with smoking, heavier caffeine use, lower mood and higher body weight. Students whose timing varied most had a clock delayed by nearly three hours. If the week left you short, a modest lie-in of under an hour plus an early night does less harm than a three-hour shift.

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The long late nap

A nap of ten to twenty minutes taken before mid-afternoon improves alertness almost immediately and does not disturb the night. Longer naps bring sleep inertia, the thick-headed half hour after waking from deep sleep, and naps late in the day take pressure off the night and delay it. After a restricted night, a ten-minute nap was the most recuperative of the lengths tested.

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Chasing sleep

Going to bed early to lie awake, drinking to knock yourself out, and scrolling until you drop all make the next night worse. Alcohol fragments the second half of the night, screens push the clock later, and time spent awake in bed teaches the brain that bed is a place for wakefulness, which is exactly what insomnia treatment works to undo. Coffee to repair the day then feeds the cycle.

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I have trained myself to need less

What people who sleep five or six hours have trained is their sense of sleepiness, not their brain. In restriction studies, subjective sleepiness levelled off after a few days while attention kept deteriorating, and in extension studies people who thought they slept enough gained alertness when allowed to sleep more. A small minority carry rare genetic short-sleep variants; the sleep societies' consensus is that seven hours is the minimum for nearly all adults.

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Polyphasic schedules unlock hours

Schedules that split sleep into several short naps to reduce total time promise more waking hours. The National Sleep Foundation convened a panel to review them and found no evidence of benefit and clear evidence of harm to sleep quality, cognition and health, because the schedules produce chronic restriction and cut the deep and REM stages. The two-week restriction data shows what that restriction does to performance. Shift workers, who live a version of this, carry the same documented costs.

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All sleep hygiene tips are equal

Sleep hygiene lists mix strong evidence with weak. A systematic review of the recommendations found good support for limiting caffeine, for regular exercise and for managing stress, and much thinner evidence in healthy sleepers for some other familiar rules. Caffeine timing and evening light have controlled trials behind them; for chronic insomnia, hygiene alone is not a treatment and cognitive behavioural therapy is.

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The four numbers

Average night is the mean length of your main sleep over the last two weeks, set against the need you chose in Goals. Regularity is the Sleep Regularity Index, from zero to one hundred; the study populations with scores under about seventy showed the delayed clocks and worse outcomes, and it needs a run of consecutive logged nights. Debt is the shortfall against your need summed over the last fourteen nights. Quality is your own one-to-five rating, the same kind of subjective score the Pittsburgh Sleep Quality Index rests on, and it matters because satisfaction is one of the dimensions of sleep health.

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Read it over weeks

One night tells you little; a fortnight tells you a lot. Chronotype is estimated from the midpoint of sleep on free days, corrected for the debt built up in the week, so it takes a couple of weekends to settle. Social jetlag needs both work days and free days logged. Give any change at least two weeks before judging it, and read the debt line rather than your mood, since the mood adapts first.

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