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What Are Sleep Stages? REM, Deep and Light Sleep Explained

Updated · 6 min read · By Taohidul Islam

Sleep stages are the different states your brain and body move through during the night. There are two main types of sleep: non-REM sleep, which has three stages (N1, N2 and N3, the last one being deep sleep), and REM (rapid eye movement) sleep, when most vivid dreaming happens. You pass through these stages in repeating cycles, usually four to six times a night.

The two types of sleep: non-REM and REM

Sleep scientists sort sleep by what brain waves, eye movements and muscle activity look like in a sleep lab. Non-REM sleep gets progressively deeper from stage 1 to stage 3. REM sleep is different: brain activity looks closer to being awake, the eyes move quickly behind closed lids, and the arm and leg muscles are temporarily relaxed so you do not act out your dreams.

A typical night moves from stage 1 to stage 2, then stage 3, then back up through stage 2 into REM, and the cycle repeats. Brief awakenings between cycles are normal, and most of them are too short to remember.

Stage 1 (N1): falling asleep

Stage 1 is the changeover from being awake to being asleep. It usually lasts only a few minutes. Your heartbeat, breathing and eye movements slow, your muscles relax (sometimes with small twitches), and your brain waves begin to slow from their daytime pattern. You can be woken easily, and some people do not feel they were asleep at all. It makes up only a small share of the night, around 5% in healthy adults.

Stage 2 (N2): light but stable sleep

Stage 2 is still light sleep, but it is deeper than stage 1. Heart rate and breathing slow further, body temperature drops and eye movements stop. Brain activity slows but shows brief bursts of electrical activity that researchers call sleep spindles. You spend more of the night in stage 2 than in any other stage, roughly 45% of total sleep for adults.

Stage 3 (N3): deep sleep

Stage 3, also called slow-wave sleep, is the deepest stage of non-REM sleep and the one you need to wake up feeling refreshed. Heartbeat and breathing reach their lowest levels of the night, muscles are relaxed and it can be hard to wake someone up. Deep sleep comes in longer stretches during the first half of the night and becomes shorter in later cycles.

Deep sleep is linked with physical restoration. According to the National Heart, Lung, and Blood Institute, deep sleep triggers the release of a hormone that supports normal growth in children and teens, and helps maintain muscle and repair cells and tissue. In adults it typically makes up as much as about a quarter of total sleep, and the amount falls with age.

REM sleep: the dreaming stage

REM sleep usually first appears about 90 minutes after you fall asleep. Your eyes move rapidly, breathing becomes faster and less regular, and heart rate and blood pressure rise toward waking levels. Most dreaming happens in REM, although some dreams occur in other stages too.

The first REM period of the night is often short, around 10 minutes. Later REM periods get longer, up to about an hour. That is why REM is concentrated in the second half of the night, and why cutting a night short tends to cost you more REM than deep sleep. REM makes up roughly a quarter of total sleep in adults. Memory consolidation most likely depends on both non-REM and REM sleep.

How long is a sleep cycle?

One full cycle through the stages takes roughly 80 to 120 minutes, often rounded to "about 90 minutes". Most adults go through four to six cycles a night. The cycles are not identical:

  • Early cycles contain the most deep sleep and only short REM periods.
  • Middle cycles have less deep sleep and longer REM.
  • Late cycles are mostly stage 2 and REM, with little or no deep sleep.

The percentages quoted above are averages from lab studies. Your own split will vary from night to night and person to person, and a sleep expert panel convened by the National Sleep Foundation found no consensus that stage percentages on their own are a good marker of sleep quality. How long you sleep, how quickly you fall asleep and how often you wake are better understood measures.

Key takeaway: deep sleep is front-loaded and REM is back-loaded. Getting enough total sleep on a regular schedule is the most reliable way to get enough of both.

How sleep stages change with age

Newborns spend a large share of sleep in REM. Deep sleep peaks in early childhood, drops sharply during the teen years and keeps declining through adulthood. A large analysis of lab sleep studies found that, as adults age, the share of deep sleep and REM tends to fall while light sleep and time awake during the night tend to rise.

This means an older adult seeing less deep sleep than they did at 25 is usually seeing a normal change, not a problem. What matters more is whether you feel rested and function well during the day.

What affects your sleep stages?

  • Total sleep time. Short nights mainly cut the REM-rich final cycles.
  • Sleep schedule. Going to bed and waking at the same times helps your body clock time the stages.
  • Alcohol. It may help you fall asleep but tends to disturb sleep later in the night. Sleep experts advise avoiding it before bed.
  • Caffeine and nicotine late in the day can make it harder to fall asleep and stay asleep.
  • Room temperature, light and noise. A dark, quiet room at a comfortable temperature supports unbroken sleep.
  • Recent short sleep. Your drive for sleep builds the longer you are awake, so after a short night or a long day you tend to sleep more deeply.
  • Medications and health conditions can change sleep structure. Talk to your doctor before changing any medication.

How sleep trackers estimate sleep stages

In a sleep lab, stages are scored from brain waves, eye movements and muscle activity using a test called polysomnography. A watch cannot measure brain waves, so it estimates stages from signals it can measure, such as movement and heart rate. Treat the result as a useful estimate rather than a lab measurement: it is most helpful for spotting trends over weeks, not for judging a single night.

Apple Watch labels sleep as Awake, REM, Core and Deep. "Core" is Apple's name for light sleep, which corresponds roughly to stages 1 and 2. The stages are saved to Apple Health, where other apps can read them.

FitRest reads those stages from Apple Health and shows them as a hypnogram alongside your heart rate, together with your restorative sleep (deep plus REM), sleep consistency and how your sleep is trending. You can see more on the sleep tracking page. FitRest does not score stages itself; they come from your Apple Watch.

When to talk to a doctor

A tracker cannot tell you whether you have a sleep disorder. See a doctor if you regularly feel unrefreshed despite enough time in bed, are very sleepy during the day, snore loudly, have been told you stop breathing or gasp during sleep, act out your dreams, or have had trouble falling or staying asleep for several weeks. A doctor may recommend a sleep study, which is how sleep stages are measured properly.

Sources

FitRest is a wellness app, not a medical device. It does not diagnose, treat or prevent any condition. Talk to a qualified health professional about any health concern.

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