Why We Sleep

Overview

Matthew Walker, a professor of neuroscience and psychology at the University of California, Berkeley, sets out what sleep does and what happens when people get too little. He covers how sleep changes across life, the purpose of dreaming, and the effects of caffeine, alcohol and screens. He argues that short sleep raises the risk of many diseases. Other researchers have said that several of his strongest claims go further than the evidence supports, so his numbers are best treated with care.

In brief

Two systems that control sleep

Walker explains that the urge to sleep comes from two separate systems. The first is the body clock, a rhythm of roughly twenty-four hours set mainly by daylight. As evening comes, the brain releases melatonin, which signals that it is time for sleep to begin. The second is sleep pressure. From the moment you wake, a chemical called adenosine builds up in the brain, and the more of it there is, the sleepier you feel. Sleep clears it.

Caffeine works by blocking the places where adenosine attaches, so the brain stops noticing its own tiredness. Caffeine also stays in the body for many hours, and a strong cup of tea or coffee in the evening can still be active at midnight.

People's clocks differ. Some are naturally early and some late, and this is partly inherited. In the teenage years the clock shifts later, so Walker argues that early school start times work against learning.

What happens during the night

Sleep is made of two main types that alternate in cycles of about ninety minutes. One is non-REM sleep, which at its deepest has slow brain waves. The other is REM sleep, when most vivid dreaming happens and the body's muscles are switched off.

The balance changes across the night. Early cycles contain more deep sleep, and later cycles, toward morning, contain more REM. This has a practical result. If you normally sleep eight hours and cut it to six by waking early, you lose a quarter of your time in bed but a much larger share of your REM sleep.

Walker's view is that the two types do different jobs, and one cannot replace the other. He also says lost sleep cannot be fully repaid. Sleeping late at the weekend recovers some of what was lost during the week but never all of it.

Sleep and memory

The part students should read most carefully is about learning. Walker describes sleep as helping memory twice. Before learning, a night of sleep refreshes the brain's short-term store, so there is room for new facts. After learning, deep sleep moves those facts into long-term storage.

Staying up all night before an exam harms both steps. The tired brain takes in less the next day, and what was studied the evening before never gets properly saved. Revising and then sleeping is, by his account, better than revising through the night.

Sleep helps physical skills too. People practising a piece of music or a movement in sport often find they perform it better after a night's sleep, with no extra practice. REM sleep adds something else. It connects new information with old in unexpected ways, which he ties to creativity and problem solving.

Dreams and emotions

Walker argues that REM sleep acts as a kind of overnight therapy. During REM the brain replays emotional memories while a stress chemical, noradrenaline, is shut off. The memory is kept, and its painful charge is reduced. This is why an upsetting event often feels more manageable after a few nights. He connects this to post-traumatic stress, where the same nightmare returns again and again, and suggests the brain is failing to finish the process.

Without enough sleep, the emotional centres of the brain react more strongly and the rational parts that calm them lose their grip. Anyone who has snapped at a family member after a bad night has felt this. A student who sleeps four hours before results day will likely take the same news harder than a rested one.

What lack of sleep does to the body

A large part of the book lists the harm linked to short sleep. Walker connects it to weaker immunity, with studies where poorly rested people responded less well to a flu vaccine. He connects it to weight gain, because tiredness changes the hormones that control hunger and fullness, so you eat more and crave sugary food. He describes how the brain clears waste products during deep sleep, including a protein associated with Alzheimer's disease.

One of his clearest examples is the seasonal clock change. In spring, when people lose an hour of sleep, heart attacks rise the next day. In autumn, when they gain an hour, the number falls.

He is equally worried about driving. A tired driver has brief lapses, a second or two long, in which the brain stops responding. At highway speed that is enough to leave the lane.

Sleeping better

Walker's advice begins with regularity. Go to bed and wake up at the same time every day, including weekends. If you follow only one rule, he says, it should be this one.

The rest concerns conditions. Keep the bedroom cool and dark. Dim the lights in the evening and put screens away before bed, since bright light delays melatonin. Avoid caffeine from the afternoon onward and avoid heavy meals late at night. Get some daylight in the morning. If you cannot fall asleep, get up and do something quiet until you feel sleepy, so that the brain does not learn to link the bed with lying awake.

He is sceptical of sleeping pills, which in his view produce sedation that lacks the benefits of natural sleep. For long-term insomnia he recommends a structured form of therapy known as CBT-I.

Try this week

1. Choose a fixed wake-up time and keep it for seven days, weekend included.
2. Have your last tea or coffee before early afternoon this week and note how quickly you fall asleep.
3. Charge your phone outside the bedroom and stop using screens thirty minutes before bed.
4. In exam week, finish revising early enough to get a full night of sleep before each paper.

If you liked this

The Sleep Revolution by Arianna Huffington is gentler. For the academic overview, the Matthew Walker lectures on YouTube.