# How sleep works: stages, body clock and how much you need

Source: https://sleepfacts.uk/how-sleep-works/
Author: Tess Marlow · Updated 2026-11-04

**Short answer:** Sleep comes in cycles of about 90 minutes, each passing through light sleep, deep sleep and dreaming (REM) sleep. When you feel sleepy depends on two things: pressure that builds the longer you are awake, and a body clock that runs on a roughly 24-hour cycle set by daylight. Most adults need 7 to 9 hours a night.

## What happens when you fall asleep?

Sleep is not one state. Through the night you move between two very different kinds of sleep:

- **Non-REM sleep**, split into three stages from light (N1 and N2) to deep (N3). Your breathing and heart rate slow, your body temperature drops and, in deep sleep, your brain produces large, slow waves.
- **REM sleep**, named after the rapid eye movements that come with it. Your brain becomes almost as active as when you are awake, most vivid dreams happen, and most of your muscles are temporarily switched off so you don't act them out.

You cycle through these stages roughly every 90 minutes. Early in the night, the cycles hold more deep sleep. Later, REM periods get longer, which is why you often wake from a dream in the morning. The chart on our [stages of sleep](/how-sleep-works/stages-of-sleep/) page shows a typical night.

## What decides when you feel sleepy?

Two systems work together, a model sleep scientists have used since the 1980s:

1. **Sleep pressure.** The longer you are awake, the more a chemical signal (adenosine) builds up in the brain, and the sleepier you feel. Sleep clears it. Caffeine works by blocking adenosine, which is why it [can keep you up](/sleep-better/caffeine/).
2. **The body clock.** A small cluster of cells in the brain keeps time on a cycle that, in lab studies, averages just over 24 hours (24.18 hours in one well-known study). It is reset each day mainly by light reaching the eyes. In the evening, as light fades, it triggers the release of the hormone melatonin, which tells the body it is night. [Your body clock](/how-sleep-works/body-clock/) explains how it works and why it matters for shift workers and travellers.

You fall asleep most easily when both line up: high sleep pressure and a body clock that says it is night.

## How much sleep do you need?

It depends mainly on age. Babies need 12 to 17 hours, school-age children 9 to 11, teenagers 8 to 10, and most adults 7 to 9. Older adults tend to sleep a little less deeply, but they don't need much less sleep. [How much sleep do you need?](/how-sleep-works/how-much-sleep-do-you-need/) has the ranges for every age, and what the studies say about sleeping too little or too much.

## Do some people naturally sleep later?

Yes. Your chronotype, whether you are a morning person or an evening person, is partly inherited and shifts with age: teenagers drift later and older adults earlier. Being an owl is not a disorder, but it can clash with school and work hours. See [larks and owls](/how-sleep-works/larks-and-owls/).

## How does sleep change as you get older?

In healthy adults, deep sleep and total sleep time fall steadily with age, and people wake more often in the night. Most of this change happens by about 60. [Sleep as you get older](/how-sleep-works/sleep-and-ageing/) explains what's normal and what isn't.

## Frequently asked questions

### How many hours of sleep does an adult need?

The NHS and the US National Sleep Foundation both say 7 to 9 hours a night for most adults, with 7 to 8 for people over 65. Some people do well on a little less or a little more. If you wake refreshed and stay alert during the day, you are probably getting enough.

### What is a sleep cycle?

One pass through the stages of sleep: from light sleep into deep sleep, back up, and into a period of REM (dreaming) sleep. Each cycle lasts roughly 90 minutes, and most people have four to six a night.

### Why do I feel sleepy in the afternoon?

Partly because of your body clock, which has a small dip in alertness in the early afternoon, and partly because sleep pressure has been building since you woke. A big lunch doesn't cause the dip but can make it easier to notice.

## References

1. [Saper CB, Scammell TE, Lu J. Hypothalamic regulation of sleep and circadian rhythms. Nature, 2005.](https://pubmed.ncbi.nlm.nih.gov/16251950/)
2. [Ohayon MM et al. Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals. Sleep, 2004.](https://pubmed.ncbi.nlm.nih.gov/15586779/)
3. [Czeisler CA et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 1999.](https://pubmed.ncbi.nlm.nih.gov/10381883/)
4. [Hirshkowitz M et al. National Sleep Foundation's sleep time duration recommendations. Sleep Health, 2015.](https://pubmed.ncbi.nlm.nih.gov/29073412/)
5. [NHS. Insomnia.](https://www.nhs.uk/conditions/insomnia/)