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The Human Body

Sleep

What happens in your body and brain overnight, and why a bad night leaves you feeling foggy.

4 min read Last updated 3 Nov 2026
A bed with a crescent moon and stars above it and small Zs drifting up.

The short version

Sleep is not a switch-off. Your brain cycles through light, deep and REM sleep several times a night. Cycles often last around 90 minutes, but timing varies. Deep sleep supports repair and growth; REM sleep is linked to memory. Your body clock and time awake help decide when you feel sleepy.

I always assumed that when I sleep my brain rests.

It doesn't. Parts of it are busier than when I'm awake.

Sleep is a different job, not a day off.

What happens as you drop off

You do not go straight from awake to asleep. As you fall asleep your heart rate and breathing slow, and your body temperature drops a little. Your brain's electrical activity changes from quick, small waves to larger, slower ones.

Sleep is divided into stages. In light sleep you are easy to wake. In deep sleep, also called slow-wave sleep, your brain waves are at their slowest and it is hard to wake you. In REM sleep, named for the rapid eye movements, your brain is highly active and most vivid dreaming takes place.

One night of sleep

  1. Start You fall into light sleep within about 15 minutes.
  2. First hour You sink into deep sleep; this stage is longer early in the night.
  3. Around 90 minutes A first REM period often begins, though timing varies.
  4. Through the night Sleep stages cycle several times, with timing that varies between people and across the night.
  5. Towards morning REM periods get longer and deep sleep shrinks.

What each stage seems to be for

During deep sleep the body releases growth hormone, which helps tissues repair and children grow. During REM sleep the brain is thought to sort the day's experiences and help store some as memories. Research also suggests that waste products which build up in the brain while you are awake are cleared more quickly during sleep, though the details are still being studied.

About a third

The share of an average adult life spent asleep. By 60, that is about 20 years.

What decides when you feel sleepy

Two things work together. The first is sleep pressure: the longer you are awake, the more a chemical called adenosine builds up in your brain, and the sleepier you feel. Caffeine works by blocking the receptors that adenosine acts on, so it hides the tiredness without removing it.

The second is the body clock, which runs on roughly a 24-hour cycle and is set mainly by light. In the evening, as it gets dark, the brain releases a hormone called melatonin, which signals that it is night. Bright light in the evening can delay the body's night-time signal; screens are one possible source of that light.

Your brain shuts down when you sleep.

The brain stays very active. During REM sleep, its activity looks similar to being awake, and parts of it work to store memories and tidy up.

Why sleeping in at the weekend only half works

Sleep pressure and the body clock both matter. If you stay up late and sleep in, you pay back some of your sleep pressure, but you also shift your body clock later, so the next night you are not sleepy at your usual time.

That is also why shift workers and people with jet lag feel so tired: their body clock and their sleep pressure are out of step. Regular times usually work better than trying to catch up.

Note: this explains how it works in general. It is not advice for your situation. For a decision about your health, money or legal position, check an official source or ask a qualified professional. If you have long-term trouble sleeping, talk to your GP.

Sources: NHS: insomnia and sleep habits; Oxford Health NHS: what happens during sleep. Most adults usually need 7 to 9 hours; sleep-stage timing varies.

Key takeaways

  • Sleep runs in cycles of about 90 minutes.
  • Deep sleep and REM sleep do different jobs.
  • The brain stays active during sleep.
  • Sleep pressure builds the longer you are awake.
  • Light sets your body clock.

Now you know.

Sleep is active, not off.

It runs in cycles through the night.

And two clocks decide when you feel sleepy.

Last reviewed 3 Nov 2026

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