Sleep

Why regularity weighs more than hours, what two short nights already do to your appetite, and the four things that decide tonight.

Kratos Natural · Domain: Sleep

The short answer: what do you change tonight?

Get up at the same time every day, weekends included. A steady rhythm counts for more than an extra hour of sleep, and the research behind that is further down this guide.

Get daylight in the morning and make the evening dark. Put your last coffee at least six hours before bed. Do not drink alcohol to fall asleep. Keep the bedroom cool and your feet warm.

If you are lying awake, do not lie there waiting. Get up, do something dull in low light, and go back once you feel sleepy.

If your sleep is troubling you, if you snore with pauses in your breathing, or if you fall asleep during the day without meaning to, see your doctor. Do not wait with it. Chapter 7 says what a doctor looks at.

What is sleep?

Sleep is not your body switched off. It is an active programme that runs in the same order every night.

You move through four stages: N1, N2, N3 and REM. The N stands for non-REM. N1 and N2 together are light sleep and fill most of the night.

N3 is deep sleep, also called slow-wave sleep, the stage with the slow brain waves (delta waves) in which the body repairs itself. REM stands for rapid eye movement, the quick movements you can see going on under the lids, and it is the stage in which most dreaming happens.

Those four make up a cycle of roughly ninety minutes, and the cycle repeats four to six times a night. The mix changes as the night goes on: deep sleep sits mostly in the first half, REM mostly in the second. Getting up two hours early therefore costs you REM above all, because the deep sleep is largely behind you by then.

What are brain waves?

Brain waves are the electrical rhythm of large groups of brain cells. You measure them with pads on the scalp, using a machine that produces an electroencephalogram, almost always shortened to EEG. What comes out is a graph with the waves running across it: small and fast while you are awake, large and slow in deep sleep.

The four stages above were worked out with that machine. The names of the sleep stages are, in effect, names of wave patterns.

Waves are sorted by speed, in waves per second (hertz). Exactly where the line between two groups falls is an agreement between researchers rather than a law of nature.

Beta waves, faster than about thirteen a second, belong to being awake and paying attention. Small and quick.
Alpha waves, roughly eight to twelve a second, appear the moment you close your eyes and do nothing. They go as you drift off.
Theta waves, roughly four to eight a second, belong to the crossing into sleep, so to N1.
Sleep spindles, short bursts of about twelve to fifteen a second, and K-complexes, single large deflections, belong to N2.
Delta waves, slower than about four a second, are the slow brain waves of N3. The more of them, the deeper the sleep.
In REM the pattern looks like being awake again, with theta alongside it, while your muscles are all but paralysed.

Those patterns also do work. In non-REM, spindles, slow waves and short sharp bursts in the hippocampus act together to fix what you learned that day. The hippocampus is the part of the brain where a new memory first sticks. In REM the theta rhythm leads instead^[59].

Delta waves are also the measuring stick for the sleep pressure in chapter 3: the longer you have been awake, the stronger they are, and across the night they fall away faster and faster^[10].

How reliable is that scoring?

Reasonably, except for the lightest stage. Two trained scorers marking up the same night separately agree closely across eleven studies. For N1 that agreement is weak^[60].

The line between just awake and just asleep cannot be drawn sharply even with pads on your head and a trained eye. A watch feeling a pulse and a movement at your wrist certainly cannot draw it sharply. It prints it every morning anyway, in minutes.

What changes about your sleep over the years?

Your sleep changes all your life, and it changes in a fixed direction. That comes from a meta-analysis: a calculation that adds the results of separate studies into one figure. This one covered 65 studies with 3,577 healthy people aged 5 to 102.

In adults, four things fall as they get older: total sleep time, sleep efficiency (the share of your time in bed that you actually sleep), the share of deep sleep and the share of REM. Two things rise: you take longer to fall asleep, and you lie awake longer once you have been asleep^[9].

After sixty, only sleep efficiency keeps falling clearly in those figures. The rest barely moves after that.

Lying awake at night is normal, then, and there is more of it as you age. An hour awake at sixty is a different thing from insomnia, and it needs no treatment as long as your days are fine.

How do you know you slept well?

By four things. You fall asleep within about half an hour. You lie awake no longer at night than is usual for your age, which is more the older you get. You get up at roughly the same time. And you get through the day without fighting to stay awake.

Half an hour is normal, then, and not a problem. That does not clash with step 8 later on, where you get out of bed if you are lying awake: that is about the nights when you notice it is not coming, not about the ordinary time falling asleep takes.

A ring or a watch is not on that list. Chapter 4 says why.

How does it work: the pressure and the clock?

Two systems decide together when you sleep: sleep pressure and the clock. The model is called the two-process model, it was put forward by Alexander Borbély, and he summed it up again himself in 2022^[10].

Sleep pressure builds from the moment you get up. The longer you are awake, the more a substance called adenosine collects in your brain. Adenosine is a waste product of your cells spending energy, and the more of it there is, the sleepier you feel. Sleeping breaks that pressure down.

Caffeine occupies the place where adenosine acts. It does not make you rested, then; it makes you blind to how tired you are.

The clock decides at which point in the day your body expects night. It sits in a small nucleus behind your eyes, does not run to exactly 24 hours, and is reset every day, mostly by light.

Everything you can do about your sleep works on one of those two. A nap takes pressure away, light moves the clock, and caffeine hides the pressure without removing it.

What exactly does light do to the clock?

Light shifts the clock, and which way depends on the time. Light early in the morning moves the clock forward, which makes you tired earlier in the evening. Light late in the evening moves it back. That effect was measured in 21 people in a laboratory, one pulse of light at a time at a different point in the day, and mapped per hour into what is called a phase response curve^[4].

You do not need a bright lamp for it. Ordinary room light in the hours before bed suppressed the production of melatonin and shortened the window in which the body made it^[5]. Melatonin is not a sleeping pill but a time signal: it tells your body that it is night.

What is your chronotype, and what does it cost you?

Your chronotype is the time your clock sleeps if left to itself, measured as the middle of your night on free days. Morning people and evening people are the two ends of that spread, and most people sit somewhere between them.

The problem is not your type but the gap between your free days and your working days. That gap is called social jet lag: you shift your sleep back and forth every week without going anywhere. Who has it worst follows straight from the definition: an evening person who has to be up early^[61].

Is food a time signal too?

Yes. Alongside light, the rhythm of eating and fasting steers your clock, and the field that studies it is called chrono-nutrition: what you eat counts, and so does when^[62].

Part of that runs through your gut flora. The bacteria in your gut keep a daily rhythm driven mostly by eating and fasting, and they in turn act on the clock genes and on sleep duration. Persistent jet lag and a diet that makes you fat flatten that rhythm, and eating inside a fixed window restores it. Much of that work comes from animal studies, so it is a mechanism rather than a prescription^[63].

Why do you have to cool down to fall asleep?

To fall asleep your core temperature has to drop, and your body does that by sending blood to your hands and feet, where the heat disappears into the bed. Your skin gets warmer, then, while your inside gets colder.

How strongly that works came out of a study in which people wore a suit that raised skin temperature by 0.4 of a degree without changing core temperature. That small amount suppressed waking in the night and shifted sleep towards the deeper stages; in older people it nearly doubled the share of deep sleep, and the chance of waking too early fell from more than half to almost none^[11]. That was a controlled suit in a sleep lab and not a duvet at home, so the direction counts and the number does not.

A hot shower works down the same route. In a meta-analysis of thirteen studies, a bath or shower at 40 to 42.5 degrees, one to two hours before bed and for as little as ten minutes, shortened the time it took to fall asleep^[12].

What is sleep for?

For three things, each shown separately.

Memory. Non-REM and REM do different jobs with what you learned that day. Non-REM sleep strengthens the memory trace; REM refines it and sharpens the difference between things that resemble each other^[13].

Clearing out. The leading idea is that during deep sleep the brain flushes waste away through the glymphatic system: a kind of rinsing in which fluid runs between the brain cells and carries waste with it. Among that waste is amyloid-beta, the protein that plays a part in Alzheimer's. That is a proposal researchers are now trying to steer and test, not an established fact^[14].

Immunity. In the first hours of the night the immune system shifts towards building memory. People who slept after a hepatitis A vaccination had a stronger and lasting immune response^[15].

What are dreams for?

Nobody knows. What is settled is that dreams mirror your day: in psychiatric patients and in people with a sleep disorder, the daytime complaints come back in the dream. And dreams carry into the next day, into mood and into creativity. What they are for is an open question^[64].

Dreams also explain something else. In chronic insomnia the measured sleep is often only mildly abnormal while the complaint is large. One explanation is that REM sleep in those people is unstable, which makes thinking during REM more like lying awake, and the night is remembered that way^[65].

What does short sleep do to testosterone?

It lowers it in young men. That comes from a short research letter in the American medical journal JAMA, about a week of restricted sleep in healthy young men. That letter carries no abstract in PubMed, so we can give you the direction and not the number^[66].

What does the research say?

How many hours of sleep do you need?

Seven to eight. Plot sleep duration against the chance of dying and you get a U: at both ends that chance is higher than in the middle.

In a meta-analysis of sixteen studies covering more than 1.3 million people, short sleepers had roughly 12% more chance of dying during the studies than people with a normal sleep duration. For long sleepers it was roughly 30% more^[1].

Those two halves are not worth the same. Behind the short half sit experiments showing that short sleep does measurable harm. Behind the long half sits mostly the reverse: illness makes people sleep longer. The dementia research shows it plainly. Only long sleep was linked to more dementia; short sleep was not^[16].

What counts for more, hours or regularity?

Regularity. More than 60,000 people wore a band on the wrist that measures movement, day and night, and such a measurement is called actigraphy. They were then followed for years without anything in their lives being changed, which is what a cohort is. How regular their sleep rhythm was predicted death more strongly than how long they slept^[2].

Catching up at the weekend does not repair it. Among 73,513 people followed for eight years, extra sleep at the weekend was not linked to less death or less cardiovascular disease, not even in those sleeping under six hours on weekdays^[17].

What do a few short nights do to you?

More than make you tired. In twelve healthy young men who slept short for two nights, the satiety hormone leptin fell by 18%, the hunger hormone ghrelin rose by 28%, and hunger rose by 24%, most of all for calorie-dense food high in carbohydrate^[18].

As far back as 1999, an experiment in eleven young men showed that six nights of four hours in bed worsened how they handled sugar, lowered the hormone that drives the thyroid and raised evening cortisol^[3].

It works the other way too. People with overweight who usually slept under 6.5 hours had one conversation about their sleep, nothing about food or exercise.

They went on to sleep a good hour longer and ate 270 kilocalories a day less, about a cheese sandwich. That is not from a food diary but measured with water whose hydrogen has been labelled, which lets you read off someone's urine how much energy they actually burned^[19].

What does that ring on your finger measure?

Less than it shows you. In a study where eleven popular sleep trackers were put alongside a full sleep study at once, in 75 people across two clinics, the agreement varied widely by device.

On a score from 0 to 1, where 1 means the tracker puts every half minute of the night in the same stage as the clinic does, the best managed 0.69 and the worst 0.26. Watches and rings were furthest out on sleep efficiency^[67].

Our advice with that: use a tracker for your rhythm and not for your night. When you go to bed and when you get up, and so how regular you are, it keeps track of well, and that is exactly the lever in step 1. The figure for deep sleep, your sleep score and your readiness score are estimates made by an algorithm, and that is what we mean above by no number from a watch or a ring: the verdicts on your night, not the times.

Another reason not to strike your own feeling off against a number: sleeping next to someone feels better and measures worse. Across the different kinds of sharing a bed, from partner to child to dog, people rated their nights better while the measured sleep was poorer^[68].

Do blue-blocking glasses help?

Yes, for falling asleep faster, and most clearly in people who have a reason to wear them. A systematic review of 29 publications on amber glasses in the evening covered sixteen studies with 453 patients between them, in which a draw decided who got the glasses and who did not. Drawing lots is what makes a study strong: the two groups then differ only in the thing you want to know about.

Across the 24 publications that were about sleep, there was substantial evidence that the glasses shorten the time to fall asleep in people with a sleep disorder, jet lag or shifting rosters. About healthy people with no complaint, the review does not say that.

The explanation is clear: less blue light means less stimulation of the cells in your retina that drive the clock, and so an earlier release of melatonin^[69].

So it is about the colour and about the amount, and the glasses only deal with the first. Wear them in a room with the ceiling light on and the amount of light stays high. Turning that lamp off costs nothing and works on both.

What does light at night do over the years?

That has now been measured on this scale for the first time. Nearly 89,000 people wore a light sensor for a week and were then followed for eight years on average. People with more light during the day had lower mortality, and people with more light at night had higher, up to 34% more risk in the brightest nights compared with the darkest half^[70].

What ruins a night, and why?

Almost everything that wrecks a night does it in one of two ways: it keeps your body alert while you want to sleep, or it moves your clock. What follows is the part that has no heading of its own anywhere else in this guide.

Worrying. Stress by itself does not keep you awake, going over it again and again does. The research calls that perseverative cognition, and across eight studies following 3,733 people over time, part of the link between felt stress and disturbed sleep ran straight through it^[116].

That link held once sleep at the start of the measurement was counted in, so it is not only the other way around.

The insomnia therapy from this chapter aims at exactly that. Across fifteen randomised trials worry dropped clearly while ruminating barely moved, and the effect was stronger on what was about sleep itself than on worrying in general^[117].

The phone within reach. Across twenty studies of 125,198 children and teenagers, a phone or tablet around bedtime went together with too little sleep, with worse sleep and with sleepiness during the day. Those who merely had the device beside them without using it slept less too^[118].

That last part is the interesting one: if not using it counts as well, screen light does not explain everything. Some of it is probably what happens on that screen, and knowing you can be reached.

That explanation has not been measured, and these are snapshots in children, so it is our judgement and not a finding.

What you eat during the day. Twenty-six adults slept in a clinic, first for four nights on a diet put together for them and then for one night after a day of choosing for themselves. After that free day falling asleep took longer and there was less deep sleep^[119].

Within that same day more fibre went with more deep sleep, and more saturated fat and more sugar went with less deep sleep and more brief awakenings. Twenty-six people is few, so this points a direction and writes no menu^[119].

Drinking in the evening. Waking up to pass urine is called nocturia, and it gets more common with the years. In a review for family doctors the first step is not a drug but the evening itself: drink less in the hours before bed, and look at what time of day a water tablet is taken^[120].

Never move a medicine yourself. That is a question for your doctor or pharmacist, timing as much as dose.

Noise. Even quiet noise causes brief awakenings you will not remember the next morning, whether it is traffic, a neighbour or a church bell^[8]. What to do about it is in step 7.

The rest each has its own piece in this guide, with the research beside it: caffeine, alcohol, nicotine, evening light, cannabis, a bedroom that is too warm, a wake time that moves and night shifts.

What does nicotine do to your nights?

Smokers sleep worse, and quitting repairs it slowly. In a review of the research on stopping, the sleep complaints held on through the first weeks of withdrawal, and those complaints predicted relapse. Those who kept it up saw recovery afterwards. In people using nicotine patches the sleep does appear to recover, though the authors call that evidence limited^[71].

What does caffeine do, and until when?

Caffeine disturbs your night for hours after you drink it. In a double-blind study, 400 mg of caffeine, about two strong cups, measurably disturbed sleep when taken at bedtime, three hours before it and six hours before it, measured with a portable sleep monitor at home. Those six hours are exactly what the common recommendation rests on^[6].

What does alcohol do to your night?

Alcohol does not help you sleep, it changes your night. A meta-analysis of 27 studies found that from about two drinks onwards REM sleep falls and starts later, and that this gets worse the more is drunk. Only from about five drinks do you fall asleep measurably faster, and that is precisely when the rest of the night is most disturbed^[20].

Does exercise help, and is the evening allowed?

Yes, and the evening is fine. In a network meta-analysis, where forms are also compared that never appeared in the same study together, 35 studies with 3,519 older adults showed several kinds of exercise improving sleep quality, and muscular endurance training combined with walking came out best^[21].

The idea that evenings are off limits does not hold. Across 28 studies with 325 participants between them, exercising in the evening did not disturb the night of healthy young and middle-aged adults, and that held for light, moderate and hard effort. Only after hard effort did REM sleep drop by about two percent^[22].

What works against insomnia?

Cognitive behavioural therapy. In the European guideline it is the first choice for long-term insomnia, with high-quality evidence. Medication is only allowed alongside it if the therapy does not work or is not available, and then for at most four weeks^[23].

In a meta-analysis of twenty studies with 1,162 people, those who had the therapy fell asleep about nineteen minutes faster and lay awake about twenty-six minutes less at night. That gain held up at later follow-up^[7].

Which parts of that therapy do the work has been picked apart in an analysis of 241 studies with 31,452 people. What pays: taking on the thoughts about sleep, mindfulness and acceptance, sleep restriction (spending less time in bed so sleep concentrates) and stimulus control (using the bed only for sleeping). What added nothing: education about sleep hygiene. Relaxation exercises were possibly even counterproductive^[24].

Treatment with a therapist in the room came out better than any form at a distance^[24].

That last point was confirmed in a study of 49 older adults with sleep problems, putting six weeks of mindfulness directly against six weeks of sleep hygiene education. The mindfulness group slept clearly better afterwards, and had less fatigue and low mood during the day too^[25].

You do not need a waiting list for it. In the records of 4,052 patients using a sleep app, that same therapy delivered through the app did better than medication after six months, and about as well as the two together. The authors still call the combination the best choice, because the app on its own held up less steadily. This is a look back at records rather than a draw^[26].

And in people with a chronic illness, such as cancer, chronic pain or after a stroke, it worked as well as it does in healthy people^[27].

What do the remedies you buy over the counter do?

Less than the packet suggests, and it differs per remedy.

Melatonin shortened the time to fall asleep in an umbrella review of several meta-analyses, measured objectively as well; light therapy and valerian did not^[28]. At the same time the European guideline advises against melatonin for treating insomnia, because the evidence is of low quality^[23]. That is not a contradiction but a difference in question: a clock that has shifted is not the same problem as insomnia as a disorder.

Magnesium in the L-threonate form kept sleep quality and daytime functioning steady in one three-week study in eighty people aged 35 to 55, while both declined in the group given a dummy, a placebo. That is a different thing from improving them. Three of the six authors work for the manufacturer^[29].

Glycine improved how people who slept badly rated their own night. That it lowers core temperature by sending more blood to the skin, the same route as a hot shower, was measured in rats and not in people. The review was written by researchers at the manufacturer^[30].

L-theanine, 200 mg a day for four weeks in thirty healthy adults with no psychiatric diagnosis, shortened the time to fall asleep and the use of sleep medication on a questionnaire. So this is self-assessment rather than measurement. Two authors work for the producer^[31].

Vitamin D improved sleep quality with moderate certainty^[32].

Do music, scent or a weighted blanket help?

All three a little, and all three with the same weakness: there is no way to test them against a placebo. Anyone who hears music, smells a scent or feels a heavy blanket knows they are getting the treatment, and that alone can colour how they judge the night.

Music before sleep reduced sleep problems in pregnant women with insomnia, across four studies with a draw, 348 participants in total. All four carried a high risk of bias^[72].

Scent helped in older adults. In a meta-analysis, aromatherapy clearly improved sleep quality, most strongly with lavender alone and with forms you do not inhale^[73].

A weighted blanket is the best supported of the three. In a study with a draw in 120 people who had, alongside their insomnia, depression, bipolar disorder, an anxiety disorder or ADHD. After four weeks, a blanket with metal chains did far better than a light one, with less fatigue and low mood during the day^[74].

Does cannabis help?

In one small study, yes. In 29 adults with insomnia, after two weeks on an oil with THC and CBD, the two active substances from the cannabis plant, 60% no longer counted as clinically insomniac, and light sleep rose by 21 minutes a night against placebo. It is a small study and the blinding fell apart in the second half^[75].

What do prescription sleeping pills do?

They work, and they differ sharply. In a network meta-analysis of 69 studies with 17,319 patients, one group came out best for falling asleep, lying awake and sleep efficiency, with few side effects: the orexin receptor antagonists, drugs that block the signal keeping you awake. The so-called Z-drugs, the family of zolpidem and zopiclone, worked as well, but with more side effects^[76].

That changes nothing about the answer above: the guideline holds medication for long-term insomnia to four weeks at most^[23].

Does sex before sleep help?

People think so, and it has not been measured. In a questionnaire of 778 adults, sex with a partner ending in orgasm went together with the feeling of a better night, equally in men and women^[77].

It is a questionnaire at one moment rather than a trial, so nobody knows whether the sex made the better night, or whether people who sleep well simply have more sex.

What does not work, or has not been studied?

Ionisers: in a meta-analysis of 33 studies there was no consistent effect on sleep, relaxation or anxiety^[33]. Mouth taping: of ten studies, two found improvement, and the authors warn about the risk of suffocation with a blocked nose^[34]. Grounding yourself while you sleep: the only study counted twelve people with no control group, and so carries no claim at all^[35].

Do the moon, the season and the clock change do anything?

The moon a little, the climate clearly, and the clock change at least not in healthy people in a measured trial.

The moon. This was found in a laboratory with no windows, where neither the volunteers nor the researchers were thinking about the moon at the time. Around full moon, delta waves fell by 30%, people took five minutes longer to fall asleep and slept twenty minutes less, with less melatonin^[78].

The climate. In a series of sleep studies in West and Central Africa, people slept seven to eight hours a day as they do everywhere on earth, counting the afternoon nap. What differed was the make-up of the night: the dry Sahelian climate gave more deep sleep than a humid or a temperate one^[79].

The clock change. At population level there is a link between the spring shift and heart attacks in the week that follows. In a measured trial in 21 healthy adults, nothing changed around either shift in arterial stiffness, sleep quality or memory. That difference is the lesson: a link in a population is about whoever has the least room, not about the average healthy volunteer^[80].

How certain is everything above?

It differs per piece, and saying so is more honest than giving it all the same confidence.

A trial with a draw is the strongest: you change one thing and hold the rest still. The pieces on light, temperature, caffeine and the therapy for insomnia rest on those.

A cohort follows people and finds association, not cause. The figures on mortality, diabetes and dementia rest on those. The clearest example of the difference is above: long sleep is associated with dementia, but the likely explanation is that the illness has been under way for years and makes people sleep longer^[16].

Who paid counts. For magnesium, glycine and theanine, authors of the study work for the manufacturer of the substance, and that is noted with each of the three above^[29]^[30]^[31].

And a guideline is a subject, not a verdict. The European one matters most here, but it was written for doctors with a diagnosed patient in front of them, and we write for somebody who sleeps badly and has been nowhere yet^[23].

Where the evidence is missing, that is here too: grounding, ionisers and white noise at home are not disproved but under-studied^[35]^[33].

Why does it matter?

What do you notice tomorrow?

Hunger, mood and concentration, and that is measurable within two nights. There is something on top of that which people underestimate: you cannot feel how unsafe you are on the road. In a cohort of 3,201 people, six hours of sleep went together with 33% more chance of a car crash than seven or eight, and severe sleep apnoea with 123% more, including in people who said they were not sleepy^[36].

What happens over years?

In a meta-analysis of 153 studies with more than five million people, short sleep was associated with 12% more death, 37% more diabetes, 17% more high blood pressure, 16% more cardiovascular disease and 38% more obesity^[37]. Below six hours, mortality rose in a straight line the shorter the sleep got.

Insomnia also tends to come before low mood. Somebody who is not depressed but sleeps badly has twice the chance of developing depression later, across 21 long-running studies^[38].

What is the story with night work and cancer?

There is a link and it is not settled. Night work that disrupts the clock has been listed since 2007 by the cancer agency of the World Health Organization as probably carcinogenic, on the strength of animal work and, at the time, eight studies in people.

Across the 26 studies that exist now, the picture points to more breast cancer, particularly after more than twenty years of night shifts or after runs of many nights in a row, but the way night work is measured differs in every one of them^[81].

The suspected mechanism is light at night suppressing melatonin, plus your internal clock and your surroundings pulling apart. On shifts, that clock turns out barely to adapt: after several nights, melatonin and cortisol still largely keep the old time^[82].

What does somebody gain by starting today?

Three things, and they reinforce each other. Sleeping longer was worth 270 kilocalories a day without any diet^[19].

In people with apnoea and overweight, twenty percent less body weight came with 57% fewer breathing pauses an hour in a model across 27 studies. That is losing a fifth of your weight, and the margins around that line are wide; half of those studies were about bariatric surgery, and the authors work largely for a drug company^[39].

And the therapy for insomnia works at any age and alongside a chronic illness as well^[23]^[27].

The plan

The order is not arbitrary: what pays most sits at the top.

Watch one thing in this chapter. Not every number below is measured. Where a number comes from research, the source is with it. Where it is a practical rule of ours, because the research gives a direction but not a figure, we say so. Both are usable, but they are not the same thing and you should be able to see which one you are looking at.

Step 1. Fix the time you get up

Pick one time and hold it seven days a week, weekends included. Getting up steers the clock and the sleep pressure at once, and it is the one lever that drags the rest along with it.

Shift by an hour at most at the weekend. That hour is our rule: the research shows regularity counts and that catching up repairs nothing, but it names no line under which you are fine^[2]^[17].

Step 2. Sort out your light

Get outside within an hour of waking, cloud included, for ten to twenty minutes. Make the last hour before bed dark: a small lamp instead of the ceiling light. Those times are ours, worked out from chapter 3: outdoor light is many times stronger than indoor light, and the clock responds to how bright it is and how late.

What a lamp does to your clock depends on how bright it is and when you have it on. Eighteen chronobiologists have set out recommendations for that in a unit that measures how much a lamp reaches your clock rather than how bright it looks, and you can read it off with an app or a light meter^[40].

Step 3. Set your caffeine line

Last coffee at least six hours before bed. Those six hours are measured; the eight we hold to when in doubt are not, and they are there because people differ in how fast they break caffeine down^[6]. Count tea, cola, energy drinks and pre-workout too.

Step 4. Sort out the temperature

Bedroom cool, duvet warm, feet warm. Take a hot shower of about ten minutes one to two hours before bed. Warm socks sound old-fashioned, but they do exactly what chapter 3 describes: they help your inside cool down.

Step 5. Take the alcohol out of your evening

If you drink, drink early in the evening and not as a sleeping aid. From about two drinks it costs you REM sleep, and the more you drink the more that becomes. Falling asleep faster only starts around five drinks, and that is precisely when the rest of your night is most disturbed^[20].

Step 6. Move, evenings included

Any kind helps. Muscular endurance training with walking alongside it came out best. Exercising in the evening is fine too: in the studies above it did not disturb the night, whatever the effort.

Step 7. Set up the bedroom

Dark, quiet and cool. Noise causes short arousals in your sleep even at low levels, whether it is traffic, a neighbour or a church bell, and it does so even when you remember nothing of it the next morning^[8].

Earplugs and an eye mask are cheap and harmless. The evidence for them comes from a meta-analysis in 1,701 patients after heart surgery, where a package of non-drug measures clearly improved sleep quality; the earplugs and the mask were part of that package and were not tested on their own^[41].

Step 8. What to do when you are lying awake

If you notice sleep is not coming and you are lying there waiting, get up. Do something dull in low light and go back once you feel sleepy. This is called stimulus control and it is one of the parts of the therapy that works^[24].

This is not the same as the first twenty minutes of your night. Taking half an hour to fall asleep is normal and needs nothing from you. It is about the waiting, not about the minutes, which is why we deliberately give no figure here: watching the clock is itself what keeps you awake.

If your thoughts are churning, write down for ten minutes during the day what is going on, so it does not have to happen in bed. Relaxation exercises are the least promising part, so if they do not work for you, that is not your fault.

Step 9. Naps, if you use them

Early in the afternoon, and short. A nap mainly improves alertness, the effect lasts about two hours, and naps before one in the afternoon came out better than later ones across those eleven studies, though that difference was not firm^[42].

Twenty to thirty minutes is the usual recommendation, to stay out of deep sleep and not wake up groggy; the naps in that meta-analysis were longer on average, at 55 minutes.

If you have to drive or work afterwards, have the coffee before the nap. In a driving simulator, twelve sleepy people made about a third of the errors with coffee alone that they made with nothing. With coffee and a short nap, only about a tenth^[43].

Step 10. After a bad night

Get up at your usual time. Eat something before you have coffee: strong coffee on an empty stomach after a broken night raised the peak in blood sugar and insulin, while the broken night on its own did not^[44]. Drink enough: short sleep is associated with a greater chance of being under-hydrated^[45]. Do not go to bed earlier and do not sleep in the next morning.

If you have to perform after a night awake, there is one substance with a signal behind it. A single high dose of creatine, 0.35 grams per kilo of body weight, improved thinking speed during 21 hours awake, and reversed part of the changes in the brain's energy household. That is one small study, and it is about an emergency rather than your ordinary week^[83].

And if snoozing is your habit?

Then it costs you about six minutes of sleep and little else. In a study with brain measurements in 31 habitual snoozers, half an hour of snoozing cost about six minutes of sleep and did not make functioning worse right after getting up; it even prevented waking straight out of deep sleep^[46].

What it does mean: snoozers sleep less on average. Snoozing is a sign of too little sleep rather than the cause of it.

One thing has to line up with step 1 though. Set your alarm for the moment you actually get out, and snooze inside that half hour. An alarm that ends at a different time every day takes away the regularity the whole plan rests on.

And if you work night shifts?

Then your clock does not come with you, however long you keep it up. After several night shifts in a row, melatonin and cortisol still largely keep the old time, and that holds both in the laboratory and in people who have done it for years^[82].

That is the most important thing to know about this, and it is also a relief. Sleeping badly after a night shift is not a matter of grit or character. Your internal clock never gets the signal that night has moved, because light and food give that signal, and for you they both fall in the wrong place.

There is another layer under it. On night work it is not only your clock and your surroundings that pull apart, but the clocks inside your body that pull apart from each other: your brain keeps the old time while your metabolism partly moves with your new mealtimes. That is part of why night work does more than tire you out^[82].

What you can do about your light. The drive home in the morning sun is the problem, not the bedroom. Light early in the morning moves your clock forward, which is exactly the wrong way for somebody about to go to sleep. That effect is mapped per hour of the day^[4]. What we take from that: dark glasses on the way home, and a dark room the moment you are in. Blackout at the window does more for your sleep than any supplement in chapter 4.

What you can do about noise. Sleeping by day means sleeping through the traffic, the neighbours and the doorbell, and noise causes short arousals even at low levels that you will not remember in the evening^[8]. Earplugs and an eye mask are best studied in hospital patients, where they were part of a package that improved sleep quality^[115]^[41]. For a day sleeper that is the same situation: surroundings not built for sleeping.

The nap before the shift, not the coffee after it. A nap takes sleep pressure away; caffeine only hides it. A nap mainly improves alertness and that effect lasts about two hours^[42]. If you have to drive after your shift, the combination is strongest: in a driving simulator, sleepy people with coffee and a short nap made about a tenth of the errors they made with nothing^[43].

And the caffeine itself. The six hours from step 3 still stand, only for you they fall in the middle of your shift. Work until six in the morning and want to sleep at eight, and your last coffee was at two. That is awkward, and it is why many night workers do fall asleep but do not stay asleep^[6].

The roster is not a detail. People differ sharply in how well they tolerate a night shift, and those very differences predict who suffers for it. A roster built per person rather than one rule for everybody is, the authors say, a promising countermeasure, though it has not been tried out^[84]. If you have any say over your roster, that is your biggest lever, bigger than everything above.

And the long-term risk, put honestly. Night work that disrupts the clock has been listed since 2007 by the cancer agency of the World Health Organization as probably carcinogenic. The 26 studies taken together point to more breast cancer, particularly after more than twenty years of night shifts, or after runs of many nights in a row^[81].

What that means for you, nobody knows exactly, and we would rather say that than offer either reassurance or a fright. The studies each measure night work differently, and a link in a population is not a prediction for one person. What is in your hands is the rest of this chapter, and that is no small consolation: light, the nap, caffeine and the roster are precisely where the most is to be won on night work.

If you sleep badly despite all of it, or fall asleep during your shift without meaning to, see your doctor and read chapter 7 first. A shifted clock asks for something different from insomnia, and falling asleep unintentionally during the day belongs with a doctor in any case.

And if you sleep next to someone?

Then you sleep measurably worse and feel better. That holds across every form: partner, child and pet in the bed. So do not start sleeping apart because a tracker or a guide says it would be better; the only thing that counts is how the two of you feel during the day^[68].

Sleeping apart is a solution rather than a defeat where there is heavy snoring or a sharply different rhythm. That is simply not tested anywhere in the literature, so it is our judgement and not a finding.

And if you have young children?

Then your problem is fragmentation and not the number of hours. In fifty mothers wearing such a band through the first four months after birth, night-time sleep stayed around 7.2 hours, while sleep efficiency crept from 79.7% in week two to 90.2% in week sixteen. The sleep was there and it was in pieces^[85].

There is a stubborn story about breastfeeding and sleep that does not hold. In a review of 35 studies, breastfed babies woke more often, but they did not sleep less in total, did not lie awake longer, and their mothers did not sleep worse^[86].

And the neighbourhood counts. Across eight studies with 67,677 children, children in poorer areas slept less. How strong that link was did depend on how sleep was measured, and on the child's sex and background^[87].

And if your teenager will not get out of bed?

Then they are fighting their own clock and the school bell, not you. In a review of 38 studies on later school start times, students slept longer as soon as school began later, even with a shift of only half an hour, and attendance, lateness, grades and the number of road crashes all moved the right way^[88].

What you can do at home is the screen. Across 52 studies in children and young people aged 3 to 17, more social jet lag went together with more screen use, weakly, and morning types were more active during the day than evening types^[89].

And if you are a woman?

Then your sleep moves with your cycle, your pregnancies and the menopause, and that is not a side issue.

The cycle. In a painless cycle with no symptoms, measured sleep stays the same, but brain activity does change: in the second half of the cycle, when progesterone is present, there are clearly more sleep spindles. With painful periods or premenstrual symptoms, sleep quality as experienced is worse^[90].

Pregnancy. In the third trimester, 42.4% of women had insomnia, across ten studies with 8,798 participants. That is almost half, and a reason to raise it with your midwife or doctor^[91].

The menopause. Sleep complaints belong to the picture, alongside the hot flushes, and how common they are differs sharply by country and population^[92]. Without hormones there is something that works: that same cognitive behavioural therapy, in four to six sessions, eases the burden of hot flushes and improves sleep^[93].

And if you are an athlete?

Then you probably need more sleep than the recommendation says, and you get less. Young athletes average about 6.3 hours against the eight to ten advised for their age, and too little sleep in them is associated with more injuries^[94].

The nap is the best studied tool for athletes. After a normal night or a bad one, a nap restores part of the performance. And unlike the advice above, in athletes the longer naps of 35 to 90 minutes usually came out better than short ones, as long as there is enough time between the nap and the effort^[95].

And if you fast, as in Ramadan?

Then you sleep about an hour less and are slightly sleepier during the day. Across 24 studies in twelve countries, total sleep time fell from 7.2 hours before Ramadan to 6.4 hours during it, and the score on a daytime sleepiness questionnaire rose by about one point^[96].

That is no reason to drop anything. It is a reason to plan the nap in that month, and not to move your wake time on top of it.

And your bedroom: air, mattress and duvet

Of those three, only temperature is properly supported, and that is above already. What else there is:

Bedding and nightwear set the climate right against your skin, and skin that gets too warm or starts to sweat measurably disturbs sleep. Which material is best has barely been studied^[97].

Ventilation sounds more logical than it turns out to be. In 36 children aged 10 to 12 sleeping in a climate chamber, the difference between fresh air and air with a lot of carbon dioxide made no difference to their thinking the next morning^[98].

Particulates outside do matter, at least for damaged lungs. In 1,308 people with COPD, the chronic lung disease in which the lungs are permanently damaged, more particulate matter around the house went together with worse sleep quality, and that link was strongest in those carrying overweight^[99].

And if you are alone a great deal?

Then there is a signal, and it comes from the fruit fly. Flies kept alone for a long time slept less and ate more, and a pattern belonging to starvation switched on in their brains while there was food in reach^[100].

That is a fly and not a person, so no advice hangs on it. What we take from it is the question rather than the answer: loneliness and sleeping badly occur together in people, and nobody knows whether one makes the other. For somebody who is alone for long stretches, that is a reason to look at the days and not only at the nights.

When should you see a doctor?

When is it sleep apnoea?

If you snore and others see you stop breathing, if you wake with a headache or a dry mouth, or if you drop off during the day. It is more common than people think: in a population study using a sleep test at home in 2,121 people, 49.7% of men and 23.4% of women had fifteen or more breathing pauses an hour. That figure is called the apnoea-hypopnoea index, the AHI: the number of times an hour your breathing stops or half falls away^[47].

The researchers note themselves that those high numbers come partly from equipment having grown more sensitive, and that the threshold for the diagnosis ought to be revised. That many people are not ill, then; what counts is whether you have symptoms with it.

In women it is missed more often, because the symptoms are different: tiredness, insomnia, headache and low mood rather than loud snoring. After the menopause it increases^[48].

What you can do yourself in the meantime: sleep on your side. In most people with apnoea the pauses happen at least twice as often on their back^[49].

A great deal is claimed about posture and build, and little is shown. What exists is this: in a hundred men with apnoea, the most severe cases had a jaw set further back, a lower-lying hyoid bone and a head held forward, compared with the milder cases and with controls^[101].

And in fifteen patients, a night-time oral appliance measurably changed the position of the neck vertebrae^[102].

Those are small studies about build and not about postural exercises. There is no evidence that you train your apnoea away with a straight back, and there is treatment that does work.

If you get a machine with a mask that holds your airway open at night, a CPAP, then the hours you actually wear it count. Across three large studies with 4,186 participants who had cardiovascular disease, the machine produced no fall in new heart problems across the whole group, but it did in those wearing it at least four hours a night^[103].

And if your child snores?

Ordinary snoring is often harmless, but snoring every night can point to apnoea, and in children that has consequences for growth, behaviour and learning. The three common causes are enlarged tonsils or adenoids, overweight and hay fever, and the first treatment is usually taking the tonsils out^[104].

When is insomnia a disorder?

When you sleep badly at least three nights a week, it has lasted at least three months, and it troubles you during the day. Those are the boundaries from the international classification that the European guideline also keeps to.

Then it is no longer a habit but a diagnosis with a treatment of its own, and that treatment is in chapter 4^[23].

Do not read that as a waiting time. Those three months say when a doctor calls it a condition, not when you are allowed to bring it to one. If you sleep badly, that is reason enough for an appointment today.

When is your clock wrong rather than your sleep?

When you sleep well but at the wrong hours. A strong evening type who has to be up at six, or somebody on shifts, does not have a sleep problem but a timing problem: the gap between the clock in your body and the clock on the wall is called social jet lag^[61].

We read that this way: then light and a different roster are the route, and a sleeping pill is not. That is our judgement and not a finding from that source.

In people who are completely blind the clock can come loose from the day altogether, because no light gets in to reset it. There is medication for that which locks the clock back on, shown in trials with a draw, though the cost of it is a barrier^[105].

In ADHD a shifted clock belongs to the picture: across 62 studies with 4,462 patients between them, an evening type and a late release of melatonin came out consistently, and there are signs that melatonin helps against the sleep complaints there^[106]. In children with autism and insomnia, prolonged-release melatonin in a study with a draw in 125 children produced nearly an hour more sleep against nine minutes on placebo^[107].

When is it something other than insomnia?

With restless legs: an uneasy urge to move your legs at rest, which eases when you walk and is worst in the evening. Have your iron checked, because two iron values in your blood belong to the first tests: ferritin, the store of iron in your body, and transferrin saturation, the share of your transport system that is filled^[50].

With dropping off during the day without meaning to, muscles going slack with emotion, or hallucinations around falling asleep: that can be narcolepsy. In the form with the slack muscles it can be shown with a test on the spinal fluid, in which the substance hypocretin is then too low^[51].

If you sleep enough and are worn out anyway, with heavy grogginess on getting up, it can be idiopathic hypersomnia: excessive sleepiness for which no cause can be found^[52].

With passing urine at night, twice or more, a bladder diary is the first step; in a review in women the causes ranged from drinking too much across the day to a bladder that holds less^[53].

With acid reflux at night, sleeping on your left side helps. In a double-blind trial with a hundred people, everybody got that advice plus a small device that buzzed as soon as they rolled onto their right side; in 44% the symptoms halved, against 24% in those whose device only pretended to work^[54].

Sleepwalking, night terrors and waking confused occur in two to four percent of adults. They belong to the deep sleep in the first half of the night. Treatment consists of removing whatever sets an episode off, because good trial material for medication is missing^[55].

What if something else is going on?

Sleep and pain push each other around. Short sleep changes the pain system itself, so that the same thing hurts more, and that pain then disturbs your sleep in turn^[56].

In cancer, up to 95% of people have sleep complaints^[57]. Three months after covid, 31% had sleep problems and 37% fatigue^[58].

In all of those cases the insomnia itself stays worth treating, because the therapy from chapter 4 works in people with a chronic illness too^[27].

And the nights you do not understand yourself?

Nightmares and sleep paralysis both come out of REM sleep. A single nightmare means nothing, but nightmare disorder is more common alongside psychiatric conditions and after trauma, and it can be treated.

Sleep paralysis, waking up while the muscle paralysis of REM sleep is still switched on, happens at least once to up to 40% of people, sometimes with very vivid hallucinations alongside it. The triggers are short sleep, an irregular rhythm and jet lag, and avoiding those is the treatment^[108].

Recurring nightmares after something shocking can be treated. In a meta-analysis, a drug and a form of imagery rehearsal, in which you play the dream out again and differently during the day, did about equally well; the combination of that rehearsal with the therapy for insomnia came out best^[109].

And if an illness is in the way?

With migraine it runs both ways. Sleeping badly is one of the best-known triggers of an attack, while sleep can also end one. The suspicion is that both come from the same place in the brain: the hypothalamus, the small region that also runs your clock, your hunger and your temperature^[110].

In dementia, up to 40% have sleep problems, and what can be done about it without medication is thinly supported. In a review by the Cochrane organisation, which sets the strictest requirements for what it counts, there were nineteen studies with a draw. Physical activity during the day, social activity and support for the carer gave some improvement, all with low certainty. There is no single approach that can be broadly recommended yet^[111].

In Down syndrome, apnoea belongs to the picture, alongside intellectual disability and a higher chance of dementia later in life^[112].

With tinnitus, treating the insomnia helps, and it helps against the ringing itself as well. In a study with a draw in 102 people with tinnitus-related insomnia, the therapy for insomnia did better than audiological care and better than a support group: more than 80% improved noticeably, against 47% and 20%. And it reduced the distress from the tinnitus itself^[113].

And with alcohol dependence it pays to treat the sleep separately. Across nine studies, treating the insomnia improved sleep quality and low mood, with behavioural therapy clearly stronger than medication. On staying off alcohol it had no demonstrable effect^[114].

And in critically ill people in intensive care it is the surroundings that do it: across twenty studies, earplugs, an eye mask, damping the noise and limiting night-time procedures improved patients' sleep quality^[115].

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