The Science of Sleep: What Your Brain Does All Night

Most people spend roughly a third of their lives asleep. For a long time this was seen as wasted time, a kind of switching off. Scientists now know that sleep is one of the most active and carefully organised things the body does. It is a state of reduced mental and physical activity in which consciousness is altered and some of the senses are partly shut down. Muscles relax, we stop interacting with the world around us, and yet the brain keeps working in complex, rhythmic patterns.

What Exactly Is Sleep?

Sleep is not the same as being unconscious. A sleeping person can still be woken by a loud noise, a bright light or someone calling their name, and the brain quietly checks incoming sounds for anything important. A mother may sleep through traffic noise yet wake at once when her baby cries. This ability to return quickly to full awareness is what separates sleep from a coma or from being under general anaesthesia, where the brain does not respond normally to the outside world.

Scientists usually describe sleep through a few key features:

• Reduced movement: the body becomes still and the muscles relax.
• A typical posture and place: humans lie down, birds may perch, bats hang upside down.
• Weaker response to stimuli: it takes a stronger signal to get our attention.
• Easy reversibility: unlike a coma, sleep ends quickly when we are woken.
• Regulation: if we miss sleep, the body tries to make up for it later.

How Scientists Discovered the Stages of Sleep

For most of history, sleep was a mystery because nobody could see what was happening inside a sleeping head. That changed with the electroencephalogram, or EEG, a device that records the brain's electrical activity through small sensors placed on the scalp. The German psychiatrist Hans Berger recorded the first human EEG in the 1920s, and researchers soon noticed that brain waves look very different during sleep than during wakefulness.

The biggest breakthrough came in 1953 at the University of Chicago. Eugene Aserinsky, a graduate student, and his supervisor Nathaniel Kleitman noticed that, at certain times during the night, sleepers' eyes darted rapidly back and forth beneath their closed eyelids. When people were woken during these periods, they usually reported vivid dreams. This stage was named REM sleep, for rapid eye movement. Everything else became known as non-REM sleep.

Today, sleep is studied in laboratories with a test called polysomnography. It records brain waves, eye movements, muscle activity, heart rate and breathing all at once, giving a detailed map of a whole night.

The Sleep Cycle: A Night in Stages

Sleep is not one uniform state. During the night, the brain moves through a cycle of stages that lasts about 90 minutes and repeats around four to six times.

Non-REM Stage 1 (N1)

This is the light, drowsy stage at the edge of sleep, lasting only a few minutes. Breathing slows, muscles begin to relax, and it is easy to be woken. Many people feel a sudden twitch at this point, called a hypnic jerk, sometimes with a feeling of falling.

Non-REM Stage 2 (N2)

Heart rate slows and body temperature drops a little. The EEG shows short bursts of fast activity called sleep spindles and large single waves called K-complexes. Scientists believe spindles help protect sleep from noise and play a part in learning. Adults spend about half the night in this stage.

Non-REM Stage 3 (N3)

Also called deep sleep or slow-wave sleep, this stage is dominated by large, slow brain waves. It is the hardest stage to wake from, and a person woken from it often feels groggy and confused for a while. Most deep sleep happens in the first half of the night. Sleepwalking and night terrors, which are more common in children, usually begin in this stage.

REM Sleep

In REM sleep the brain becomes almost as active as when awake. The eyes move quickly, breathing and heart rate become irregular, and most vivid, story-like dreams occur. At the same time, nearly all the body's muscles are temporarily paralysed, except those that control the eyes and breathing. This paralysis is thought to stop us from acting out our dreams. REM periods grow longer as the night goes on, which is why people often wake from a dream in the early morning.

The Two Forces That Control Sleep

Why do we get sleepy at night and wake up in the morning? Sleep researchers often explain it with the two-process model, proposed by the Swiss scientist Alexander Borbély in the early 1980s. It describes two systems working together.

Sleep Pressure

The longer we stay awake, the stronger the urge to sleep becomes. One chemical linked to this pressure is adenosine, which builds up in the brain during waking hours and is cleared during sleep. Caffeine, found in coffee, tea and many soft drinks, makes us feel more alert largely because it blocks the brain's adenosine receptors. It hides tiredness for a while but does not remove the need for sleep.

The Body Clock

The second system is the circadian rhythm, an internal clock that runs on a cycle of roughly 24 hours. The word comes from the Latin circa diem, meaning about a day. The master clock is a tiny cluster of nerve cells in the hypothalamus called the suprachiasmatic nucleus, which receives signals about light directly from the eyes. When it gets dark, the pineal gland releases the hormone melatonin, which tells the body that night has come. Morning light suppresses melatonin and helps reset the clock each day.

The importance of this clock was recognised with the 2017 Nobel Prize in Physiology or Medicine, awarded to Jeffrey Hall, Michael Rosbash and Michael Young. Working with fruit flies, they uncovered the genes and proteins that make cells keep time. Almost every cell in the human body has its own small clock, all coordinated by the master clock in the brain.

Jet lag shows what happens when the two systems fall out of step. After a long flight across time zones, the body clock still follows the old schedule, so travellers feel sleepy at midday and wide awake at night until the clock gradually adjusts to the local daylight.

Why We Need Sleep

Scientists are still investigating all the functions of sleep, but several are well supported by research.

• Memory and learning: during sleep, the brain replays and strengthens what was learned during the day. The hippocampus, a region important for forming new memories, appears to pass information to the cortex for long-term storage. Students who sleep after studying generally remember more than those who stay up.
• Cleaning the brain: a 2013 study in mice found that the spaces between brain cells widen during sleep, allowing fluid to wash away waste products more efficiently. This cleaning network is called the glymphatic system.
• Growth and repair: much of the body's growth hormone is released during deep sleep, which matters especially for children and teenagers. Tissues and muscles also repair themselves.
• Immune defence: people who sleep too little tend to catch infections more easily, and vaccines can work less well in people who are short of sleep.
• Emotional balance: REM sleep seems to help the brain process emotional experiences. After a bad night, people are usually more irritable and less able to cope with stress.

How Much Sleep Do We Need?

The amount of sleep a person needs changes a great deal with age. Sleep medicine organisations give general guidelines like these:

• Newborns sleep for most of the day, around 14 to 17 hours, in many short periods.
• Children aged 6 to 12 need about 9 to 12 hours a night.
• Teenagers aged 13 to 18 need about 8 to 10 hours.
• Adults need at least 7 hours a night.

Teenagers face a special challenge. During puberty, the body clock shifts later, so melatonin is released later in the evening. Many teenagers genuinely cannot fall asleep early, yet school often starts early in the morning. This is why many sleep scientists support later school start times for older students.

Babies also spend far more of their sleep in REM than adults do: about half of a newborn's sleep is REM, compared with roughly a fifth to a quarter in adults. Researchers think this may support the rapid development of the young brain.

What Happens Without Enough Sleep

Even one short night has effects. Attention, reaction time and judgement get worse, and people may experience microsleeps, brief lapses of a few seconds in which the brain falls asleep without the person noticing. Microsleeps are particularly dangerous for drivers. Some research has found that driving after a long period without sleep can impair performance about as much as driving after drinking alcohol.

Over the long term, regularly sleeping too little is linked to a higher risk of obesity, type 2 diabetes, heart disease and depression. Lack of sleep also affects appetite hormones, which is one reason tired people often crave sugary or fatty food.

One famous case is that of Randy Gardner, a 17-year-old high school student in San Diego who in late 1963 and early 1964 stayed awake for about 11 days as a science fair project. As the days went on he had trouble concentrating, remembering and speaking clearly. After his record he slept for about 14 hours and recovered. Guinness World Records later stopped accepting attempts to stay awake because of the risks to health.

Common Sleep Disorders

Sleep medicine recognises dozens of sleep disorders. Some of the best known are:

• Insomnia: difficulty falling asleep or staying asleep, often linked to stress, worry or irregular habits.
• Obstructive sleep apnea: the airway repeatedly narrows or closes during sleep, interrupting breathing. Loud snoring and daytime tiredness are typical signs.
• Narcolepsy: a condition in which people fall asleep suddenly during the day. It is usually caused by a loss of brain cells that produce a chemical called orexin, also known as hypocretin, which helps keep us awake.
• Restless legs syndrome: an uncomfortable urge to move the legs, usually worse in the evening.
• Sleepwalking and night terrors: episodes that arise from deep non-REM sleep. They are common in childhood and usually fade with age.

Sleep in the Animal Kingdom

Sleep, or something very like it, has been observed in mammals, birds, reptiles, fish and even insects such as fruit flies. In 2017, researchers reported a sleep-like state in the upside-down jellyfish Cassiopea, an animal with no brain at all. This suggests that sleep is extremely ancient in evolutionary terms.

Animals have found remarkable ways to sleep:

• Dolphins and other whales rest one half of the brain at a time. This is called unihemispheric sleep, and it lets them keep swimming and come to the surface to breathe.
• Some birds, such as ducks, can also sleep with one half of the brain, keeping one eye open to watch for predators. Frigatebirds have been recorded sleeping briefly while flying.
• Giraffes and elephants sleep only a few hours a day in the wild.
• Bats and koalas are among the sleepiest mammals, spending most of the day asleep or resting.

Sleep in History and Culture

People have always wondered about sleep. In Greek mythology, Hypnos was the god of sleep and the twin brother of Thanatos, the god of death. One of Hypnos's sons was often said to be Morpheus, who shaped dreams, and the painkiller morphine was named after him because of its sleep-inducing effect. The Romans called the god of sleep Somnus, which is the origin of words such as insomnia.

Sleep habits have also changed over time. The historian Roger Ekirch found many references in old European writings to a first sleep and a second sleep, separated by a period of wakefulness in the middle of the night. Before artificial lighting became widespread, people often went to bed soon after dark and might get up for an hour or so to pray, read, tend the fire or talk before sleeping again. Electric lighting, and later screens, pushed bedtimes later and turned sleep into a single block for most people.

Simple Habits for Better Sleep

Sleep scientists agree on several practical habits that help most people sleep better:

1. Keep a regular schedule: going to bed and waking up at similar times every day, including weekends, keeps the body clock steady.
2. Get daylight in the morning: bright light early in the day helps set the circadian rhythm.
3. Limit screens before bed: bright screens can delay melatonin release, and exciting content keeps the mind alert.
4. Watch the caffeine: its effects can last for several hours, so a drink in the afternoon can still disturb sleep at night.
5. Make the bedroom dark, quiet and cool: a slightly cooler room helps, because body temperature naturally falls as we drift off.
6. Be active during the day: regular exercise helps people fall asleep faster and sleep more deeply.

Sleep is not a pause in life but a vital part of it. Every night, while we lie still, the brain sorts memories, the body repairs itself and the internal clock prepares us for a new day.