Brain health and sleep

You spend about a third of your life asleep, and the first REM period of a night usually starts about 90 minutes in.

Why the brain needs sleep

Sleep takes about one third of our time, and NINDS calls quality sleep as essential to survival as food and water. Without it, the brain cannot form or maintain the pathways that let you learn and make new memories, and it becomes harder to concentrate and respond quickly. The brain stays active during sleep. Recent findings suggest that sleep also does housekeeping, removing toxins that build up in the brain while you are awake. Its full biological purpose is still a research question, and this lesson describes what is known in general terms; it is not advice for any person's health.

The parts of the brain that run sleep

Several structures share the work. Inside the hypothalamus, the suprachiasmatic nucleus receives information about light directly from the eyes and keeps your daily rhythm. The brain stem controls the switch between waking and sleep, and during REM sleep it sends signals that relax the muscles of posture and the limbs, so you do not act out dreams. The thalamus, the sensory relay, goes quiet in most stages of sleep, letting you tune out the world, but becomes active in REM sleep. The pineal gland raises its output of the hormone melatonin when it gets dark. During the day a chemical called adenosine builds up and makes you feel sleepy; caffeine works by blocking adenosine.

A night in stages

Sleep comes in two basic types: non-REM sleep, with three stages, and rapid eye movement (REM) sleep. Stage 1 is the brief changeover from waking, lasting several minutes. Stage 2 is light sleep; heartbeat and breathing slow, body temperature drops, and you spend more of the night here than in any other stage. Stage 3 is the deep sleep you need to feel refreshed, and it comes in longer periods during the first half of the night. REM sleep first appears about 90 minutes after falling asleep. The eyes move quickly behind closed lids, brain waves look closer to waking, breathing becomes faster and irregular, and the arm and leg muscles are temporarily paralysed. Most dreaming happens in REM. Through the night you cycle through these stages several times, and REM periods get longer toward morning.

Two internal systems decide when you sleep. The circadian rhythm, run by the body's roughly 24-hour clock, makes you sleepy at night and helps you wake in the morning. Sleep-wake homeostasis keeps count of how long you have been awake: the drive to sleep grows stronger every waking hour, which is why you sleep longer and more deeply after a short night. Light has the greatest influence on the clock, which is why night shifts and jet lag can make sleeping at the right time hard.

MedlinePlus lists what too little sleep does: it can impair your ability to think clearly, react quickly and form memories, which raises the chance of mistakes and accidents. It can affect mood, and over time poor sleep is linked with a higher risk of problems such as high blood pressure, heart disease, stroke, obesity and type 2 diabetes. The same page also lists everyday sleep habits. This course does not repeat them, because it explains how sleep works and does not give health advice.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.