# The stages of sleep: what happens in light, deep and REM sleep

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

**Short answer:** Sleep has four stages: two of light sleep (N1 and N2), deep sleep (N3) and REM sleep, when most dreaming happens. You cycle through them about every 90 minutes, four to six times a night. Deep sleep comes mostly in the first half of the night and REM in the second. In adults, light N2 sleep makes up about half the night.

## What are the four stages of sleep?

Sleep scientists divide sleep into stages using brain waves, eye movements and muscle tone recorded overnight in a sleep lab.

| Stage | What happens | Share of a typical adult night |
| --- | --- | --- |
| N1 (light) | The drowsy border between waking and sleep. Muscles relax and you may twitch or feel as if you're falling. Easy to wake from. | About 5% |
| N2 (light) | Heart rate and temperature drop. The brain produces short bursts of activity called sleep spindles. | About half |
| N3 (deep) | Large, slow brain waves. Hardest stage to wake from. Most of it comes in the first few hours. | Roughly 15–25%, less with age |
| REM | Brain nearly as active as when awake, eyes dart about, vivid dreams, most muscles switched off. | Roughly 20–25% |

The shares are averages from sleep-lab studies of healthy people, pooled in a large review in 2004. They vary from person to person and night to night, and they change with age.

## How does a night move through the stages?

You don't go through the stages once. You fall into N1, then N2, then deep sleep, come back up through N2 and have a period of REM. That is one cycle, and it lasts about 90 minutes, though anything from 70 to 120 is normal. Most people have four to six cycles a night.

The cycles aren't identical. The first one or two hold most of the night's deep sleep. As the night goes on, deep sleep shrinks and REM periods lengthen, so the longest dreams come towards morning. Short awakenings between cycles are normal, and most people don't remember them. The chart at the top of this page shows a typical pattern.

## What is deep sleep for?

Deep sleep is when the body releases most of its growth hormone, and it seems to matter for restoring the brain after a day awake. It also plays a part in turning the day's experiences into long-term memories: during deep sleep, the brain replays recently learned information, and experiments that boost or disrupt slow waves change how much people remember. See [sleep and memory](/benefits/memory-and-learning/).

Deep sleep pressure builds the longer you are awake. After a night of poor sleep, the next night usually holds more deep sleep, which is the body catching up.

## What is REM sleep for?

Less is known for certain. REM sleep seems to be involved in processing emotions and some kinds of learning, particularly skills and finding connections between ideas. The temporary paralysis that comes with it stops you acting out dreams. When that system fails, people can shout, punch or kick in their sleep, a condition called REM sleep behaviour disorder that is worth [seeing a GP](/sleep-problems/when-to-see-a-gp/) about.

## What affects your sleep stages?

- **Age.** Deep sleep falls steadily through adult life. See [sleep as you get older](/how-sleep-works/sleep-and-ageing/).
- **Alcohol.** A drink before bed tends to increase deep sleep in the first half of the night and fragment the second half. See [alcohol and sleep](/sleep-better/alcohol/).
- **Sleep loss.** After a short night, the brain prioritises deep sleep, then catches up on REM.
- **Some medicines,** including many antidepressants, reduce REM sleep.

## Can you trust your sleep tracker's stages?

Consumer watches and rings don't measure brain waves. They estimate stages from movement and heart rate. They are reasonably good at telling whether you are asleep or awake across a night, but much less reliable at telling light, deep and REM sleep apart. Treat the stage breakdown as a rough guide, and don't let a low "deep sleep" score worry you if you feel rested.

## Frequently asked questions

### How much deep sleep do you need?

There is no agreed target. In healthy young and middle-aged adults, deep (N3) sleep usually makes up around 15 to 25% of the night, and the share falls with age. If you wake refreshed, you are almost certainly getting enough, whatever a tracker says.

### Are sleep trackers accurate about sleep stages?

Only roughly. Watches and rings estimate stages from movement and heart rate rather than brain waves. They are reasonably good at telling sleep from wake over a night, but much less reliable at separating light, deep and REM sleep, so don't worry about night-to-night changes in your deep sleep score.

### Why do I wake up during a dream?

REM periods get longer towards morning, so if you wake naturally, you are often in or just out of REM. Brief awakenings between cycles are normal and usually forgotten.

### What happens if you wake up during deep sleep?

You are likely to feel groggy and confused for a few minutes or longer. This is called sleep inertia. It is one reason long naps, which reach deep sleep, can leave you feeling worse than short ones.

## References

1. [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/)
2. [Rasch B, Born J. About sleep's role in memory. Physiological Reviews, 2013.](https://pubmed.ncbi.nlm.nih.gov/23589831/)
3. [Saper CB, Scammell TE, Lu J. Hypothalamic regulation of sleep and circadian rhythms. Nature, 2005.](https://pubmed.ncbi.nlm.nih.gov/16251950/)
4. [Milner CE, Cote KA. Benefits of napping in healthy adults. Journal of Sleep Research, 2009.](https://pubmed.ncbi.nlm.nih.gov/19645971/)