Study topic · about 3 minutes

Sleep and the brain

Sleep is run by two systems in the brain: a body clock in the hypothalamus that keeps a roughly 24-hour rhythm, and a sleep drive that builds up the longer you stay awake. Each night you cycle through three stages of non-REM sleep and a period of REM sleep, four to six times, one cycle about every 90 minutes.

Key points

The circadian rhythm is a roughly 24-hour cycle kept by a small area of the hypothalamus, the suprachiasmatic nucleus, which daylight resets through the eyes.
The pineal gland releases melatonin in the dark, telling the body it is night.
Non-REM sleep has three stages; stage 3 is deep, slow-wave sleep.
REM sleep is when most vivid dreams happen: the brain is very active, but most muscles go limp.
Sleep helps store memories and restore the body; missing it harms attention, mood and health.

The body clock: the circadian rhythm

Deep in the hypothalamus, just above where the optic nerves cross, sits a tiny cluster of about 20,000 neurons called the suprachiasmatic nucleus. It is the brain's master clock. Light reaching special cells in the eyes resets it every morning. At night, the clock signals the pineal gland to release melatonin, a hormone that makes you sleepy.

Jet lag and shift work confuse the clock: your body is on one schedule while the sun and your alarm are on another.

The stages of sleep

Stage	What happens	Brain waves (EEG)
Non-REM stage 1	Light sleep as you drift off. You may feel like you are falling, or see images (hypnagogic sensations).	Slower theta waves
Non-REM stage 2	You are truly asleep: body temperature drops and the heart slows. Much of the night is spent here.	Sleep spindles and K-complexes
Non-REM stage 3	Deep, slow-wave sleep. It is the hardest stage to wake from, and when the body repairs itself.	Slow, large delta waves
REM	The eyes dart, the brain is about as active as when awake, and most muscles go limp. Vivid dreams happen here.	Fast, wake-like waves

Early in the night, cycles have more deep stage 3 sleep; toward morning, REM periods get longer. If you miss REM sleep, you get extra REM sleep the next night, which is called REM rebound.

Which parts of the brain run sleep?

Hypothalamus: holds the master clock, and groups of cells that switch sleep on and off.
Pineal gland: releases melatonin at night.
Thalamus: during sleep it blocks most sensory signals from reaching the cortex, and it helps make sleep spindles.
Pons: switches on REM sleep and, with the medulla oblongata, keeps the muscles limp during it.
The reticular activating system, a network running through the brainstem, keeps you awake and alert.

Why do we sleep?

Sleep seems to do several jobs at once. The memory consolidation theory says sleep helps store and organize the day's memories: the hippocampus replays recent experiences during deep sleep. The restoration theory says sleep restores the body and brain, for example by clearing waste products that build up while you are awake.

Theories of dreaming include activation-synthesis, which says dreams are the cortex trying to make sense of random signals from the brainstem during REM sleep, and consolidation, which says dreams help process and store memories.

Common sleep disorders

Insomnia: trouble falling or staying asleep.
Narcolepsy: sudden attacks of sleep during the day, sometimes with sudden muscle weakness. It is linked to the loss of orexin (hypocretin) cells in the hypothalamus.
Sleep apnea: breathing stops again and again during sleep, often with loud snoring.
REM sleep behavior disorder: the muscles fail to go limp during REM sleep, so people act out their dreams.
Sleepwalking (somnambulism): walking or doing things while in deep non-REM sleep.

Quick quiz

5 questions. Your answers are saved and come back in your daily review.

Which structure holds the brain's master clock?
Show the answer
Hypothalamus. The suprachiasmatic nucleus of the hypothalamus keeps the circadian rhythm.
Which gland releases melatonin at night?
Show the answer
Pineal gland. The pineal gland releases melatonin in the dark.
In which stage of sleep do most vivid dreams happen?
Show the answer
REM sleep. Dreaming mostly happens in REM sleep, when the brain is very active.
What happens to most muscles during REM sleep?
Show the answer
They go limp. The pons and medulla keep most muscles limp, so we do not act out our dreams.
Which sleep disorder makes breathing stop again and again during sleep?
Show the answer
Sleep apnea. In sleep apnea the airway closes or breathing pauses, over and over.

In these study guides

MCATSleep, drugs, memory, language and reward
AP Psychology1.5 Sleep
Intro psychologySleep and drugs (sections 4.2 to 4.5)

Sources

How Sleep Works: Sleep Phases and Stages, National Heart, Lung, and Blood Institute (NIH)

Spotted a mistake? Tell us.