Why Is Sleep Important? An Interactive Explainer

Why is sleep important? The short answer

Sleep is important because your body and brain do essential maintenance work while you sleep that they can't do as well while you're awake. You spend about a third of your life asleep, and good sleep is as essential to survival as food and water.¹ During sleep your body repairs tissues and releases growth hormone, your immune system gets to work, your brain sorts and stores new memories, and research suggests the brain clears away waste that builds up during the day.¹²¹⁰

Sleep affects almost every system in your body: brain, heart, lungs, metabolism, immune function and mood.¹ That's why too little sleep makes it harder to focus, learn and react, makes you more likely to catch colds, and over time raises the risk of problems like high blood pressure, diabetes, obesity and depression.⁴⁸

One honest note: scientists still don't fully understand the biological purpose of sleep.¹ But they have found several clear jobs it does. Think of sleep as a night shift, not an off switch.

What happens to your body when you sleep?

A key time for growth and repair is deep sleep (also called slow-wave sleep or stage 3). It comes in longer stretches during the first half of the night and is the stage you need to feel refreshed.¹

Your heart rests. In non-REM sleep, your blood pressure and heart rate fall, so your heart doesn't work as hard as when you're awake.²
You grow and repair. A big pulse of growth hormone is released soon after deep sleep begins. In a classic 1968 study, this peak lasted 1.5 to 3.5 hours and was delayed if falling asleep was delayed.¹⁵ Growth hormone supports growth and the repair of muscles and other tissues, which is one reason sleep matters so much for children and teens.²²⁵
Your immune system gets busy. Some immune cells work harder while you sleep, which is why people who don't sleep enough may be more likely to catch colds and other infections.²²⁷ In one study, 164 adults wore sleep-tracking wristbands for a week and then were given a cold virus in the lab. Those who slept less than six hours a night had about four times the odds of developing a cold compared with those who slept more than seven.¹⁶
You save energy. Your energy use drops during sleep, by as much as about 10 percent in humans, and your body temperature falls a little too.²⁶

Does your brain clean itself while you sleep?

This is one of the most exciting discoveries in sleep science, and it's still being worked out. Brain tissue itself doesn't contain the ordinary lymph vessels that clear waste from the rest of your body. Instead, a clear liquid called cerebrospinal fluid flows around the brain and spinal cord.¹⁰¹¹

In 2013, scientists at the University of Rochester watched this fluid in the brains of living mice. When the mice were asleep, the spaces between their brain cells grew by about 60 percent, and the fluid flowed through much faster, washing away waste. That waste included beta-amyloid, a protein linked to Alzheimer's disease, which was cleared faster during sleep.¹⁰¹¹ The researchers called this plumbing the glymphatic system. "We need sleep. It cleans up the brain," said Maiken Nedergaard, the study's senior researcher.¹¹

What about people? The evidence is growing. A 2019 brain-imaging study found that during non-REM sleep, slow brain waves are followed by pulses of cerebrospinal fluid washing through the human brain.¹² A small 2021 study found that one night without sleep slowed the clearance of a tracer substance from people's brains.¹³ But not every result agrees: a 2024 study in mice found that brain clearance was actually lower during sleep.¹⁴ So "sleep helps clean the brain" is a strong, exciting idea, while exactly how it works is still being tested. NIH scientists describe it as "recent findings suggest that sleep plays a housekeeping role."¹

How does sleep help memory and learning?

Sleep helps you learn and form long-term memories.² Without it, you can't form or maintain the brain pathways that let you learn and create new memories.¹ Here's the current picture:

Collect. During the day, new memories are held for a while in the hippocampus, a seahorse-shaped part of the brain.
Replay and move. In deep sleep, the brain replays recent memories and gradually moves them into long-term storage in the cortex, the brain's outer layer.¹⁷
Steady. REM sleep, the dreaming stage, may help stabilise those memories.¹⁷ Memory saving most likely needs both non-REM and REM sleep.¹

Sleep before learning matters too. In one experiment, after a single night without sleep, people's hippocampus was less active while they tried to learn, and they remembered less afterwards.¹⁹ So an all-nighter before a test can backfire. Sleeping on it really helps.

Curious about the stages themselves, and why REM is when the most vivid dreams happen? See our explainer How do dreams work?, which lets you scrub through a whole night's sleep cycles. And if you've ever woken up and found you couldn't move for a moment, that's linked to REM sleep too: see What causes sleep paralysis?

How does sleep affect your mood?

Without enough sleep, you might feel frustrated, cranky or worried, and find it harder to judge other people's emotions.⁴ REM sleep seems to help here: in a UC Berkeley study, REM sleep was linked to the brain's emotion centre, the amygdala, reacting less strongly the next day to upsetting things people had seen, as if sleep took some of the sting out.¹⁸ Getting enough sleep can help reduce stress and improve your mood.⁵

How does sleep work? What makes you sleepy

Two systems work together to decide when you sleep.¹

Sleep pressure (sleep-wake homeostasis). Your drive to sleep gets stronger every hour you're awake. A chemical called adenosine, a by-product of your cells using energy, builds up and makes you sleepy. Caffeine works by blocking adenosine.¹
Your body clock (circadian rhythm). This internal clock runs on a cycle of roughly 24 hours. A small cluster of cells in the brain, the suprachiasmatic nucleus, gets information about light directly from your eyes. When it gets dark, it signals the pineal gland to make more melatonin, the hormone that helps you fall asleep.¹

Light is perhaps the biggest influence on your sleep timing, and light in the evening can make it harder to fall asleep.¹ In a Harvard sleep-lab study, people who read on a light-emitting tablet before bed took longer to fall asleep, made less melatonin, had their body clocks shifted later and were less alert the next morning than when they read a printed book.²¹

Why do teenagers stay up late?

It's not just habit. During the teen years, the body clock naturally shifts later.²⁰ Melatonin starts being released later in the evening and sleep pressure builds more slowly, so many teens simply don't feel tired until 11 pm or later. Left to their own schedule, many would sleep from about 11 pm or midnight until 8 or 9 am.²⁴ Early school start times clash with this, which is why the American Academy of Sleep Medicine says middle and high schools should start at 8:30 am or later.⁹

How much sleep do you need?

How much sleep you need depends on your age. These are the recommendations from the American Academy of Sleep Medicine, used by the CDC and NIH:⁸³⁹

Age	Recommended sleep (per 24 hours)
Babies 4–12 months	12–16 hours (including naps)
Children 1–2 years	11–14 hours (including naps)
Children 3–5 years	10–13 hours (including naps)
Children 6–12 years	9–12 hours
Teens 13–18 years	8–10 hours
Adults	7 hours or more (NHLBI: 7–9 hours)

Needs vary from person to person, and there's no magic number that works for everyone of the same age.¹ Sleeping more than 9 hours isn't necessarily harmful, and can help young adults and people who are sick or catching up on sleep.³ Many teens fall short: about three in four U.S. high school students get less than 8 hours on school nights.⁹⁶

Sleep cycles: why the last hours count

Sleep comes in cycles of roughly 90 minutes (a full cycle takes about 1.5 to 2 hours), and a night usually has four to six of them.²⁵ Deep sleep is packed into the early cycles, while REM periods get longer toward morning.¹ So when you cut a night short by setting an early alarm, you don't lose an "average" slice of sleep. You lose a big share of the night's REM. In our simplified model in the player, waking at 5 instead of 7 after a 10 pm bedtime cuts sleep by about a fifth but removes over 40 percent of the night's REM.

What happens if you don't get enough sleep?

Regularly sleeping less than you need is linked to problems with attention, behaviour and learning, and a higher risk of accidents, injuries, high blood pressure, obesity, diabetes and depression.⁸ In the short term you may have trouble learning, focusing and reacting.⁴

Sleep debt adds up. In a lab study, adults who spent six hours in bed each night for two weeks developed steadily worse attention. By the end they did about as badly as people who'd gone up to two nights without any sleep, yet they rated themselves only slightly sleepy.²²
Driving drowsy is dangerous. Sleepiness slows reactions even if you don't nod off. Drowsy driving was involved in about 91,000 U.S. crashes in 2017.⁷
Sports suffer, and improve. When Stanford basketball players aimed for 10 hours in bed each night for several weeks (sleeping almost two hours more), their sprints got faster and their free-throw and 3-point shooting percentages rose by about 9 points.²³

Can you catch up at the weekend? Sleeping in may help a little, but it may not fully make up for lost sleep.¹ And sleeping very late can push your body clock later still.²⁴

How to get better sleep

Habits recommended by the CDC:⁵

Go to bed and get up at the same time every day, even at weekends.
Keep your bedroom quiet, relaxing and cool.
Turn off electronic devices at least 30 minutes before bedtime.
Avoid caffeine in the afternoon and evening, and big meals before bed.
Get regular exercise.

Getting outside into daylight in the morning also helps set your body clock.²⁵ If you often have trouble sleeping, or feel sleepy during the day even after enough sleep, talk to a parent or doctor.⁵

Common misconceptions

"Your brain switches off when you sleep." Your brain and body stay remarkably active during sleep, doing repair, clean-up and memory work.¹

"You can train yourself to need less sleep." Research shows you can't get by on little sleep without effects, and short sleepers often don't notice how impaired they are.⁴²²

"Teens who stay up late are just lazy." The teenage body clock really does shift later.²⁰

"You can fully catch up at the weekend." Extra weekend sleep may not be enough to make up for a week of short nights.¹

"Six hours is fine for a teenager." Teens need 8 to 10 hours.⁸

FAQ

What are 5 reasons why sleep is so important?

Sleep helps your body repair and grow (growth hormone is released in deep sleep), supports your immune system so you get sick less often, helps you learn and save memories, keeps your mood steadier, and keeps you alert and safe, for example when playing sports or driving.

Why do we actually need sleep?

Scientists are still working out every reason, but sleep clearly does jobs the body can't do as well while awake: deep sleep supports growth and tissue repair, the sleeping brain sorts and stores memories, and research in mice suggests the brain clears waste faster during sleep. Without enough sleep, thinking, mood and health all suffer.

How much sleep do teenagers need?

The American Academy of Sleep Medicine recommends 8 to 10 hours per 24 hours for teens aged 13 to 18, and 9 to 12 hours for children aged 6 to 12. Adults should get at least 7 hours.

Is it normal to need 9 hours of sleep?

Yes. For teenagers, 9 hours is right in the middle of the recommended 8 to 10 hours. Some adults need more than 7 to 8 hours too, and sleeping more than 9 hours isn't necessarily harmful, especially for young adults or people who are sick or catching up on sleep.

Is 6 hours of sleep enough?

For most people, no. It's well below the 9 to 12 hours children aged 6 to 12 need, the 8 to 10 hours teens need, and the 7 hours recommended for adults. In a lab study, adults allowed only 6 hours in bed each night for two weeks got steadily worse on attention tests, ending up about as bad as after a whole night without sleep, and didn't realise how much worse they'd got.

What happens if you don't get enough sleep?

In the short term you may find it harder to focus, learn and react, feel more cranky or worried, and be more likely to get colds. Drowsiness is dangerous when driving. Over the long term, too little sleep is linked to health problems such as high blood pressure, diabetes, obesity and depression.

Can you catch up on sleep at the weekend?

Sleeping in can help a bit, but it may not fully make up for a week of short nights, and very late weekend mornings can make it harder to feel sleepy at your usual bedtime. A regular sleep schedule works best.

Why is sleep important?

Run the night shift yourself

Play with the night helpers!

The night crew

Sleep is a night shift

Night helpers

Awake or deep sleep?

Deep sleep and repair

Day or night?

Wash the brain

The glymphatic rinse

A brain bath

Sleep on it

Consolidation

Save your memories

When do you get sleepy?

Two-process model

Build your night

Build a night

Your night