Sleep and Dreams
We spend about a third of our lives asleep. Far from switching off, the brain uses that time to store memories, clean itself and reset the body.
Every night, without being taught, you lie down, close your eyes and lose consciousness for hours. Most animals sleep in some form, from fruit flies to whales. Scientists once thought sleep was just the brain switching off. In fact the sleeping brain is busy with work that it cannot do while you are awake.
Your body clock
Deep in the brain, just above where the nerves from your eyes cross, sits a tiny cluster of cells that acts as a master clock. It runs on a cycle of about 24 hours, called a circadian rhythm, and it sets the timing of body temperature, hormones, hunger and alertness.
Light is the clock's main signal. Bright morning light tells it the day has begun. As evening falls and light dims, a gland in the brain releases a hormone called melatonin, which helps make you sleepy. Screens and bright lights late at night can delay melatonin and push your body clock later. That is one reason teenagers, whose clocks naturally shift later, often find early mornings so hard. Jet lag happens when you cross time zones faster than the clock can adjust.
The stages of sleep
Sleep is not one steady state. Through the night you pass through cycles of about 90 minutes, repeated four to six times. Each cycle begins with light sleep, when you can be woken easily. Then comes deep sleep, when brain waves become slow and large, breathing and heart rate fall, and it is hard to wake up. Most deep sleep happens in the first half of the night.
Each cycle ends with REM sleep, named after the rapid eye movements that happen during it. In REM sleep the brain becomes almost as active as when you are awake, but the muscles of the body are temporarily paralysed, apart from those for breathing and moving the eyes. REM periods get longer towards morning.
Why we dream
Most vivid, story-like dreams happen during REM sleep, although some dreaming happens in other stages too. Dreams often mix recent events, old memories, emotions and pure invention. The paralysis of REM sleep stops you acting out your dreams. Nobody knows exactly why we dream. Leading ideas suggest that dreaming helps process emotions, rehearse responses to threats, and link new memories with older ones, but the question is still open.
What sleep does
Sleep strengthens memory. During deep sleep, the brain replays what you learned during the day and moves it into longer-term storage; studies show people remember facts and skills better after a night's sleep. Sleep also seems to clean the brain: fluid flows more freely through the sleeping brain, washing away waste products that build up during waking hours. During deep sleep the body releases growth hormone and repairs tissues, and the immune system works more effectively.
When sleep goes wrong
Missing sleep has quick effects. After a bad night, attention, mood and reaction times all suffer; staying awake for a full day can slow reactions as much as alcohol. Over months and years, too little sleep is linked to weight gain, weaker immunity, heart disease and poorer mental health. Common sleep problems include insomnia, difficulty falling or staying asleep, and sleep apnoea, when breathing repeatedly stops during the night.
Sleeping well
Good habits help: going to bed and getting up at similar times each day, getting daylight in the morning, keeping the bedroom dark, cool and quiet, and putting screens away before bed. Caffeine can keep you awake for hours after a drink, so it is best avoided later in the day. Sleep is not wasted time; it is part of how a healthy brain and body work.
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Test yourself
4 questions · answers explained at the end
1.Which hormone helps make you sleepy in the evening?
2.In which stage of sleep do most vivid dreams happen?
3.About how long is one sleep cycle?
4.What does sleep do for memory?