What Happens in the Brain When You Sleep?
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For a third of your life your brain is doing something that looks, from outside, like nothing. Inside, it is one of the busiest periods of the day. Sleep runs on a cycle of stages with different electrical rhythms and different jobs, the brain washes out the chemical debris of waking, and the memories of the day are replayed and reorganised. Miss it and every one of those processes shows up as a symptom.
The ninety-minute cycle
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
Filing the day
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.
Washing the brain
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.
The dreaming brain
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.
What too little does
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.
The takeaway
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.