What Happens in the Brain When You Sleep?

Sleep is not one state but a sequence. After falling asleep the brain descends through three stages of non-REM sleep, each with slower and larger electrical waves on an EEG, reaching deep slow-wave sleep within about half an hour. It then climbs back up and enters REM sleep, named for the rapid eye movements that accompany it, in which the brain's activity looks almost like waking while the body's muscles are paralysed. The whole cycle takes around ninety minutes and repeats four or five times a night.

The proportions shift as the night goes on. Early cycles are dominated by deep sleep and late ones by REM, which is why a person who sleeps five hours instead of eight loses mostly REM, and why the longest and strangest dreams come just before waking. The stages, in order:

• •Stage 1: drifting off, a few minutes, easily woken
• •Stage 2: light sleep with bursts of activity called spindles, about half the night in total
• •Stage 3: deep slow-wave sleep, hardest to wake from, the stage that restores the body
• •REM: vivid dreaming, an active brain, a paralysed body

New memories are fragile. During the day they are held largely by the hippocampus, a structure that records experiences quickly but has limited capacity, and during sleep they are moved and rewritten. Recordings from rats show the hippocampus replaying the day's activity during slow-wave sleep, in the same sequence of neuron firings that occurred when the animal ran a maze, only faster, and the replay is coupled to bursts of activity in the cortex, where long-term memories are stored. People who learn a task and then sleep perform better on it the next day than people who stay awake for the same interval.

REM sleep seems to do something different, strengthening the emotional and associative side of memory and possibly stripping the emotional charge from difficult experiences so that the memory remains without the distress. The two stages appear to divide the work, which is one reason both are needed and neither can be skipped without cost.

In 2013 researchers at the University of Rochester found that during sleep the spaces between brain cells in mice widen by about sixty percent and cerebrospinal fluid flows through them, flushing out waste products that build up during waking. The system, named the glymphatic system, clears among other things the protein amyloid beta, which accumulates in the brains of people with Alzheimer's disease. The finding gave a physical answer to the question of why every animal with a brain sleeps: the brain cannot clean itself while it is running at full power.

The pressure to sleep also has a chemical clock. Adenosine, a by-product of the brain's energy use, accumulates during the day and makes the brain sleepier the longer it is awake; sleep clears it. Caffeine works by blocking adenosine's receptors, which is why it postpones sleepiness rather than removing the need.

In REM sleep the brain's visual, emotional and memory regions are highly active while the prefrontal cortex, which handles logic and self-monitoring, is quiet, which fits the character of dreams: vivid, emotional, bizarre and accepted without question until waking. The brainstem sends a signal that paralyses the body's voluntary muscles, so that dreams are not acted out. When that mechanism fails, in REM sleep behaviour disorder, people punch and kick in their sleep, and the condition is now known to be an early sign of Parkinson's disease.

Why the brain dreams at all is still argued about. Theories include memory processing, emotional regulation, rehearsal of threats and a kind of overnight simulation that keeps the visual system tuned in the dark. What is settled is that dreaming is not incidental; adults deprived of REM sleep show a rebound, entering it faster and staying in it longer on the following nights, as if catching up on something the brain requires.

The effects of short sleep are measurable within a day. Reaction times slow, attention lapses, and the ability to judge one's own impairment goes first, so that a person after a night of five hours feels adequate and performs like someone mildly drunk. The amygdala, which processes emotion, becomes more reactive and the prefrontal control over it weaker, which is why tired people are irritable. Over weeks, blood sugar control worsens, appetite hormones shift towards eating more, and immune responses weaken. Over years, chronic short sleep is associated with heart disease, diabetes and dementia, though which way the causation runs is not fully known.

Most adults need between seven and nine hours, and the number who genuinely function well on less is far smaller than the number who believe they do.

Sleep pressure from adenosine explains why you get tired the longer you are awake, but not why you can get a second wind in the evening after a poor night, yet feel at your worst in the small hours. That comes from a second system, the circadian clock, run by a small cluster of cells in the hypothalamus called the suprachiasmatic nucleus. It keeps a rhythm of roughly twenty-four hours and is reset each day by light reaching special cells in the retina, which is why morning daylight helps you wake and bright screens late at night can push bedtime later.

In the evening the clock signals the pineal gland to release melatonin, a hormone that tells the body it is night rather than knocking you out like a sleeping pill. Sleep comes most easily when the two systems line up, with plenty of adenosine built up and the clock set to night. Jet lag and shift work are what happens when they do not. Adolescence also moves the clock later, by an hour or two, so teenagers who struggle to fall asleep before midnight and wake at seven are often fighting their biology rather than being lazy.

Some animals have found a way round the danger of being unconscious. Dolphins, many birds and some seals can sleep with one half of the brain at a time, a state called unihemispheric slow-wave sleep. One hemisphere shows the slow waves of deep sleep while the other shows waking activity, and the eye linked to the waking half often stays open. A dolphin can keep swimming and surfacing to breathe in this way, and fur seals use it when they are in the water but sleep with both halves at once on land, where they are safer.

Birds take it furthest. Recordings from great frigatebirds, which stay over the ocean for weeks at a time, showed that they sleep in flight in bouts lasting seconds, sometimes with one hemisphere and sometimes with both, adding up to well under an hour a day, compared with more than twelve hours when they are back on land. Humans show a faint echo of the same strategy. On the first night in an unfamiliar place, one hemisphere of the brain stays in lighter sleep and reacts more to unusual sounds, which may be why people so often sleep badly in a new hotel room.

While you are awake, a substance called adenosine builds up in the brain, and the longer you stay awake, the more of it accumulates, creating what researchers call sleep pressure. Sleep clears it again. Caffeine works by blocking the receptors adenosine acts on, so you feel less tired even though the pressure is still building underneath.

Caffeine lasts longer than many people realise. Its half-life in adults is commonly around five hours, though it varies widely between people, so half of an afternoon coffee can still be active in the evening. Studies have found that caffeine taken even several hours before bed can reduce sleep quality and the amount of deep sleep, which is why sleep researchers suggest keeping caffeine to the morning and early afternoon.

A sleeping brain cycles every ninety minutes through deep slow-wave sleep and active REM sleep, replaying and filing the day's memories, flushing out chemical waste through widened spaces between cells, and dreaming with the body safely paralysed. Each stage has a job, which is why cutting sleep short costs attention, mood and, over time, health.