Knowledge Atlas

Body & Health · Depth 2 · Introductory · 2 min read

Sleep

Why your body needs sleep, what happens in each stage of the night, and how much sleep people of different ages need, according to the NIH.

Evidence Level 2 · AuthoritativeOpen on the map

This page is general education, not professional advice. For decisions about your health, safety, money or legal situation, consult a qualified professional.

On this page
  1. Why sleep matters
  2. Your body clock
  3. The stages of a night’s sleep
  4. How sleep changes with age
  5. How much sleep do you need?
  6. When to get help
  7. Real-life examples
  8. Evidence & sources

Why sleep matters

Sleep is important because it affects many of your body’s systems. Not getting enough sleep, or enough good-quality sleep, raises your risk of heart and breathing problems, and affects your metabolism and your ability to think clearly.[1]

Your body clock

You have internal body clocks that decide when you’re awake and when your body is ready for sleep. These clocks run on cycles of about 24 hours, and they are kept in step by light, darkness and your sleep schedule.[1]

The stages of a night’s sleep

There are two basic types of sleep: non-REM and REM (rapid eye movement) sleep. Once you fall asleep, you move through them in a predictable pattern.[2]

  • Non-REM stage 1: the transition between being awake and being asleep.[2]
  • Non-REM stage 2: you are now asleep.[2]
  • Non-REM stage 3: deep sleep, also called slow-wave sleep. You usually spend more time in it early in the night.[2]
  • REM sleep: your eyes twitch and your brain is active. Dreaming usually happens now, and your muscles normally go limp so that you don’t act out your dreams.[2]

The whole cycle starts over every 80 to 100 minutes, and there are usually four to six cycles a night. REM sleep tends to get longer later in the night.[2]

How sleep changes with age

Newborns spend more time in REM sleep. Slow-wave sleep keeps decreasing through adulthood, and older people may not have any at all.[2]

How much sleep do you need?

The U.S. National Heart, Lung, and Blood Institute gives these amounts:[3]

AgeRecommended sleep
Under 4 monthsVaries widely
4–12 months12–16 hours per day
1–2 years11–14 hours per day
3–5 years10–13 hours per day
6–12 years9–12 hours per day
13–18 years8–10 hours per day
Adults7–9 hours per night

Adults who regularly sleep less than 7 hours a night may have more health problems. Sleeping more than 9 hours can be helpful for young adults, people recovering from lost sleep and people who are ill.[3]

When to get help

If you’re worried that you sleep too much or too little, talk to a healthcare provider.[3]

Real-life examples

  • Why dreams feel so real

    Dreaming usually happens during REM sleep. During REM your muscles normally go limp, which stops you from acting out your dreams.[2]

  • The deep sleep of early night

    You usually spend more time in deep (slow-wave) sleep early in the night, and more time in REM sleep later on.[2]

  • Babies sleep a lot

    Babies aged 4 to 12 months need about 12 to 16 hours of sleep per day, and newborns spend more of their sleep in REM.[3, 2]

  • Sleep changes with age

    Deep, slow-wave sleep keeps decreasing through adulthood, and some older people may have none at all.[2]

Learn more

Short descriptions are our own summaries. The resources belong to, and are run by, their publishers.

  • GuideFreeBeginner
    How Sleep Works (NHLBI) ↗

    by NIH

    The U.S. National Heart, Lung, and Blood Institute’s plain-language guide to sleep, sleep stages and how much sleep you need.

Evidence & sources

Level 2 · Authoritative

Supported by highly reputable institutions such as government agencies, universities or standards bodies.

Why this level? The information comes from the U.S. National Heart, Lung, and Blood Institute, part of the NIH. Health guidance can be updated as research continues, and this page is educational, not medical advice.

This is a Knowledge Atlas editorial classification of the sources we could find, not a certificate of truth. How we evaluate knowledge

Sources

Based on 3 sources from 1 institution: NIH.

Show all 3 sourcesHide the list
  1. AuthoritativeNational Institutes of Health· Government agencyHow Sleep Works (National Heart, Lung, and Blood Institute)Opened and checked against this page on 28 Sept 2026
  2. AuthoritativeNational Institutes of Health· Government agencySleep Phases and Stages (National Heart, Lung, and Blood Institute)Opened and checked against this page on 28 Sept 2026
  3. AuthoritativeNational Institutes of Health· Government agencyHow Much Sleep Is Enough? (National Heart, Lung, and Blood Institute)Opened and checked against this page on 28 Sept 2026