Why Do We Need Sleep?

What happens during the stages of sleep?

People spend about one-third of their time asleep, and the brain and body stay active during that time.^[1] Sleep comes in two basic types: rapid eye movement (REM) sleep and non-REM sleep, which scientists divide into three stages, each linked to particular brain waves.^[1]

Deep sleep, the stage needed to feel refreshed in the morning, comes in longer periods during the first half of the night.^[1] The first REM period begins about 90 minutes after falling asleep.^[1] A typical night cycles through non-REM and REM sleep several times, with REM periods growing longer later in the night.^[1]

What does sleep do for memory and the body?

Everyone needs sleep, yet its biological purpose remains a mystery, according to the U.S. National Institute of Neurological Disorders and Stroke (NINDS).^[1] Sleep affects almost every tissue and system in the body.^[1]

More than a century of research has established that sleep helps people retain memories.^[6] Current theories, described in a 2013 review, hold that recently learned memories are reactivated during slow-wave sleep, which corresponds to non-REM stage 3, and transformed so they can be built into long-term memory; the REM sleep that follows may stabilise them.^[6] NINDS adds that memory consolidation most likely requires both non-REM and REM sleep.^[1]

During sleep, the body works to support healthy brain function and maintain physical health, and in children and teenagers it also supports growth and development.^[3] Sleep, especially slow-wave sleep, works with the body clock to boost the release of growth hormone and lower levels of the stress hormone cortisol.^[6] A particular type of immune cell also works harder while people sleep.^[3]

Does sleep clean waste out of the brain?

NINDS describes recent findings suggesting that sleep plays a housekeeping role, removing toxins that build up in the brain during waking hours.^[1] A 2013 study of live mice, published in Science, found that natural sleep or anaesthesia came with a 60% increase in the space between brain cells, which increased the exchange of fluid in the brain and sped up the clearance of β-amyloid, a protein linked to neurodegenerative diseases.^[7] The fluid pathways involved had been named the glymphatic system.^[7] The authors suggested that the restorative effect of sleep may come from removing waste that builds up while awake.^[7]

The idea is disputed. A 2024 study in Nature Neuroscience, also in mice, measured the movement and clearance of fluorescent molecules in the brain and found that clearance was markedly reduced, not increased, during sleep and anaesthesia.^[8] Its authors wrote that their results challenge the idea that the core function of sleep is to clear toxins from the brain, while noting that larger molecules might behave differently from the small dye they used.^[8] They call the question unresolved, with published findings on both sides.^[8]

What makes us feel sleepy?

Two internal systems work together to regulate sleep: sleep-wake homeostasis and the circadian rhythm.^[1]

Homeostasis tracks the need for sleep, a drive that grows stronger with every hour awake and, after a period without sleep, makes people sleep longer and more deeply.^[1] A compound called adenosine is linked to this need: its level in the brain keeps rising while a person is awake, signalling a shift toward sleep.^[2] Caffeine counteracts sleepiness by blocking adenosine.^[1]

The second system is the body's internal clock.^[1] The suprachiasmatic nucleus, clusters of cells in the hypothalamus, receives information about light directly from the eyes and controls the body's behavioural rhythm.^[1] Most people's natural circadian cycle is slightly longer than 24 hours, and cues such as light and darkness keep it in step with the day.^[2] The hormone melatonin starts to rise in the evening and peaks in the early morning, and it is thought to promote sleep.^[2] Jet lag and night shift work can put this internal clock out of step with the actual time of day.^[1]

How much sleep do adults need?

In 2015 the American Academy of Sleep Medicine (AASM) and the Sleep Research Society issued a joint consensus statement recommending that adults sleep 7 or more hours per night on a regular basis to promote optimal health.^[5] A panel of 15 experts reached this after reviewing 5,314 scientific articles, and the recommendation covers adults aged 18 to 60.^[5] It adds that more than 9 hours a night may be appropriate for young adults, people recovering from sleep debt and people who are ill.^[5]

The U.S. Centers for Disease Control and Prevention (CDC) gives these figures for adults.^[4]

Other official figures differ: NINDS states that most adults need 7–9 hours a night, older people included, while the CDC table gives 7–8 hours from age 65 and no upper limit from 18 to 60.^[1][4] The AASM statement notes that individual sleep need is influenced by genetic, behavioural, medical and environmental factors.^[5]

What happens without enough sleep?

A lack of sleep makes it harder to concentrate and respond quickly.^[1] The AASM statement lists outcomes associated with regularly sleeping less than 7 hours, including weight gain and obesity, diabetes, high blood pressure, heart disease, stroke, depression and a higher risk of death, as well as weaker immune function, more pain, more errors and a greater risk of accidents.^[5]

Studies have also found that too little good-quality sleep can raise levels of the hormones that control hunger, reduce the body's response to insulin and lead people to eat more.^[3] People who do not sleep enough may be more likely to catch colds and other infections.^[3]

Cause and effect are not always clear: NINDS notes that it is still an open question whether lack of sleep leads to certain disorders or whether certain diseases cause a lack of sleep.^[1]