Why do we sleep? What the brain does at night

Why we sleep has no single answer yet, but the main reasons are clear: sleep lets the brain sort and store what it learned, restores the body and brain chemistry, and keeps nearly every system in the body working well. Two systems decide when sleep comes, a pressure that builds the longer you are awake and an internal 24-hour clock. Going without enough sleep harms attention, mood and long-term health.

In short

• A night's sleep is a series of roughly 90-minute cycles through light, deep and REM sleep, with more deep sleep early in the night and more REM late.
• Sleep is timed by sleep pressure, which builds with each hour awake, and by the circadian clock, which follows daylight.
• Sleep strengthens memories and supports the immune system, the heart and the brain's chemistry; regularly sleeping under 7 hours is linked to a long list of health problems.

What happens during a night's sleep?

Sleep isn't one state but two kinds, which alternate through the night. Non-REM sleep has three stages. In the first, lasting a few minutes, breathing and heart rate slow and the brain's electrical waves begin to slow. The second is light sleep, marked by brief bursts of brain activity, and takes up more of the night than any other stage. The third is deep, slow-wave sleep, when the brain's waves are slowest and it is hardest to wake someone.

REM sleep, named for the rapid eye movements that come with it, first arrives about 90 minutes after falling asleep. The brain becomes nearly as active as when awake, breathing and heart rate rise, and most vivid dreaming happens. According to the US National Institute of Neurological Disorders and Stroke (NINDS), signals from the brainstem relax the muscles of the limbs during REM, so that people don't act out their dreams.

A worked example shows how the stages are arranged. Take someone who sleeps from 11 p.m. to 7 a.m.:

Cycle	Roughly	What dominates
1	11:00 p.m. to 12:30 a.m.	A long stretch of deep sleep, a short REM period at the end
2	12:30 to 2:00 a.m.	Deep sleep again, slightly more REM
3	2:00 to 3:30 a.m.	Mostly light sleep, deep sleep shrinking, REM growing
4	3:30 to 5:00 a.m.	Light sleep and a long REM period, little deep sleep
5	5:00 to 7:00 a.m.	Light sleep and the longest REM periods of the night

The pattern has a practical consequence. Deep sleep is packed into the first half of the night and REM into the second. Someone who cuts their night from 8 hours to 6 by setting an earlier alarm loses mostly REM sleep, not an even slice of everything.

What decides when we fall asleep?

Two systems work together, an idea set out by the Zurich sleep researcher Alexander Borbély in 1982 as the two-process model. The first is sleep pressure. As the brain works through the day, a chemical called adenosine builds up between its cells and makes you feel sleepier the longer you stay awake; sleep clears it. Caffeine works by blocking adenosine's receptors, which hides the pressure without removing it.

The second is the circadian clock, a group of thousands of cells in the hypothalamus called the suprachiasmatic nucleus. It takes light signals directly from the eyes and keeps a roughly 24-hour rhythm, raising alertness in the day and signaling the pineal gland to release the hormone melatonin as darkness falls. That clock is why jet lag happens, why night-shift work is hard, and why the hour shift of daylight saving time can disturb sleep for days. Normally the two systems line up: pressure is highest at night, just as the clock stops pushing for wakefulness.

What does sleep actually do?

Sleep is not the brain switching off; it is the brain doing work that it can't do as well while awake. Scientists have strong evidence for several jobs, and they are not mutually exclusive.

Memory is the best supported. During sleep the brain replays and strengthens what it learned during the day, moving some memories from short-term storage in the hippocampus, a region of the brain for new memories, toward longer-term storage in the cortex. Deep sleep seems especially important for facts and events, and people who sleep after learning recall more than those who stay awake for the same time.

Restoration is the second. Sleep supports the immune system, the repair of tissue and the regulation of hormones that control appetite and blood sugar. Heart rate and blood pressure fall during non-REM sleep, giving the cardiovascular system a regular rest.

The third, and newest, is cleaning. In a 2013 study in Science, Lulu Xie, Maiken Nedergaard and colleagues at the University of Rochester found that in sleeping mice, the spaces between brain cells widened by about 60%, and beta-amyloid, a protein that builds up in Alzheimer's disease, was cleared about twice as fast as in awake mice. They proposed that fluid flushing through the brain, the "glymphatic system," removes waste built up during the day. NINDS describes this housekeeping role as a recent finding while noting that sleep's biological purpose "remains a mystery."

Still argued

Whether sleep really speeds the brain's waste clearance is contested. In 2024, a team led by Nick Franks and Bill Wisden at Imperial College London reported in Nature Neuroscience that clearance of a dye from the brain was about 30% lower in sleeping mice than in mice kept awake, and about 50% lower under anesthesia, the opposite of the 2013 result. Supporters of the glymphatic idea question the newer methods, and its critics question the older ones. Both studies were in mice, and whether either applies to people is unknown. No one in the debate disputes that sleep matters; what is argued is this one mechanism.

What happens when we don't sleep enough?

The short-term effects are familiar and measurable: slower reactions, poorer attention and memory, irritability, and more errors and accidents. After a full day and night without sleep, people perform some tasks about as badly as people who have been drinking. Over time, the effects add up. The American Academy of Sleep Medicine and the Sleep Research Society concluded in a 2015 consensus statement that adults should sleep 7 or more hours a night regularly, and that sleeping less is linked to weight gain and obesity, diabetes, high blood pressure, heart disease and stroke, depression, weaker immunity and a higher risk of death.

Many people fall short. In a US survey by the Centers for Disease Control and Prevention, the share of adults sleeping under 7 hours in 2022 ranged from 30% in Vermont to 46% in Hawaii, depending on the state. How much sleep someone needs varies with age: children and teenagers need more than adults.

The links between short sleep and disease come mostly from studies that follow people over time, so some of the connection may run the other way, with illness disturbing sleep. But experiments that restrict sleep in healthy volunteers for a few nights show early changes, such as worse blood sugar control, that point in the same direction.

This explains; it isn't medical advice

Ongoing trouble falling or staying asleep, loud snoring with pauses in breathing, or sleepiness that interferes with daytime life can be signs of a sleep disorder such as insomnia or sleep apnea. These are common and treatable, and a doctor is the right person to see.

Check yourself

• Without looking back: what are the two systems that decide when you get sleepy, and how does each work?
• Why does waking up two hours early cost mostly REM sleep rather than deep sleep?
• What did the 2013 and 2024 mouse studies each find about the brain's waste clearance during sleep?

Sources

• Brain basics: understanding sleep — National Institute of Neurological Disorders and Stroke, 2025
• Sleep drives metabolite clearance from the adult brain — Xie et al., Science, 2013
• Scientists find sleep may not clear brain toxins as previously thought — UK Dementia Research Institute, on Miao et al., Nature Neuroscience, 2024
• Recommended amount of sleep for a healthy adult — American Academy of Sleep Medicine and Sleep Research Society, Sleep, 2015
• FastStats: sleep in adults — Centers for Disease Control and Prevention, 2024