How Does Sleep Work? Cycles, Stages and What Changes With Age

Sleep is not one steady state. The brain moves through stages during the night, and the pattern changes across a lifetime. This guide sets out what the U.S. National Heart, Lung, and Blood Institute (NHLBI) and a meta-analysis of sleep studies say about the stages, the cycles, what times sleep, and what changes with age.

It is not medical advice. If sleep trouble is persistent, talk to a healthcare provider.

The Short Answer

Sleep has two phases: REM and non-REM. The cycle starts over every 80 to 100 minutes, usually four to six cycles a night, and you may wake briefly between cycles.
Non-REM sleep has three stages. Stage 3 is deep sleep, or slow-wave sleep, and you usually spend more time in it early in the night. REM sleep is more common later in the night.
Two systems help time sleep. One is a need for sleep that builds while you are awake. The other is the body clock, which follows a rhythm of about 24 hours and responds to light.
The pattern changes with age. Slow-wave sleep peaks in early childhood, drops sharply in the teenage years and keeps decreasing through adulthood.
The numbers are ranges. NHLBI gives 80 to 100 minutes for a cycle and four to six cycles a night.

What Happens During Sleep

NHLBI says that when you sleep you cycle through two phases, rapid eye movement (REM) and non-REM sleep. Sleep studies use sensors to record eye movements and brain activity, and those recordings are used to classify the phases and stages (NHLBI, sleep phases and stages, last updated March 2022).

Non-REM sleep has three stages:

Stage 1 is the transition between wakefulness and sleep.
Stage 2 is where you are asleep.
Stage 3 is called deep sleep or slow-wave sleep, after a pattern that appears in measurements of brain activity. You usually spend more time in this stage early in the night.

During REM sleep, your eyes twitch and your brain is active, with activity similar to that of your waking hours. Dreaming usually happens during REM sleep. Your muscles normally become limp so that you do not act out your dreams. You usually have more REM sleep later in the night. NHLBI adds that you do not have as much REM sleep in colder temperatures, because during REM sleep your body does not regulate its temperature properly. Our guide on body temperature and sleep covers that.

What Times Your Sleep

NHLBI describes two systems (NHLBI, sleep/wake cycle).

The body clock. Your body has several internal clocks, called circadian clocks, that typically follow a 24-hour repeating rhythm. The central clock, in the brain, tells you when it is time for sleep. It stays in step with the environment through light, darkness and other cues. NHLBI says most people’s natural circadian cycle is slightly longer than 24 hours, and that some people naturally wake early and others stay up late. The light-dark cycle influences when the brain makes melatonin, which starts to rise in the evening and peaks in the early morning. As light increases, the body releases cortisol, which prepares it to wake up. NHLBI notes that bright artificial light in the late evening can disrupt this process. We explain the clock in our guide on the circadian rhythm, and what the research says about cortisol in our guide on cortisol and sleep.

The need for sleep. Your body has a biological need for sleep that increases the longer you have been awake. NHLBI links it to a compound called adenosine, whose level in the brain rises while you are awake and signals a shift toward sleep. Caffeine and certain drugs can interrupt this by blocking adenosine. Our guide on sleep pressure goes through what is known and unknown about it.

How Sleep Changes With Age

NHLBI says the patterns and types of sleep change as people mature. Newborns spend more time in REM sleep. The amount of slow-wave sleep peaks in early childhood and then drops sharply in the teenage years. It continues to decrease through adulthood, and older people may not have any slow-wave sleep at all (NHLBI, sleep phases and stages).

NHLBI’s page on the sleep/wake cycle adds that it is natural for many teens to prefer later bedtimes and to sleep later in the morning than adults. It says neurons that promote sleep are lost as part of normal aging, which makes it harder for older adults to stay asleep, and that other factors, such as less physical activity or less time outdoors, also affect circadian rhythms. As a result, it says, older adults usually sleep less and wake up earlier (NHLBI, sleep/wake cycle).

A 2004 meta-analysis pooled 65 studies of healthy people aged 5 to 102 (3,577 people) whose sleep was recorded overnight. In adults, total sleep time, sleep efficiency, the percentage of slow-wave sleep, the percentage of REM sleep and the time it takes to reach REM sleep all decreased with age. Time to fall asleep, the percentage of stages 1 and 2, and time awake after falling asleep increased with age. Only sleep efficiency continued to decrease after age 60. The authors add that the size of these differences changed depending on how carefully participants had been screened for mental disorders, organic disease, drug or alcohol use, sleep apnea and other sleep disorders (Ohayon et al., 2004).

What Is Uncertain

Cycle length is a range. NHLBI gives 80 to 100 minutes and four to six cycles a night. We did not find a fixed number that applies to every person or every night.
Age patterns are averages. The 2004 review covers studies published from 1960 to 2003, and its authors say the age differences are smaller or larger depending on how well participants were screened.
We did not review consumer devices. Watches, rings and apps that report “deep” and “REM” minutes are outside this guide, and we make no claim about how accurate they are.
The NHLBI pages we read show a last-updated date of March 2022.

Questions

What are the stages of sleep?

Non-REM sleep has three stages (1, 2 and 3, with stage 3 called deep or slow-wave sleep), and REM sleep is the second phase. Sleep studies classify them from eye movements and brain activity.

How long is a sleep cycle?

NHLBI says the cycle starts over every 80 to 100 minutes, with usually four to six cycles a night.

Which stage is deep sleep?

Stage 3 of non-REM sleep, also called slow-wave sleep. You usually spend more time in it early in the night.

Do you dream only in REM sleep?

NHLBI says dreaming usually happens during REM sleep. It does not say that it happens only then.

Does the amount of deep sleep change with age?

Yes. NHLBI says slow-wave sleep peaks in early childhood, drops sharply in the teenage years and continues to decrease through adulthood, and older people may not have any.

Sources Cited in This Guide

National Heart, Lung, and Blood Institute, NIH. “How Sleep Works: Sleep Phases and Stages.” Last updated March 24, 2022. Read September 20, 2026. NHLBI
National Heart, Lung, and Blood Institute, NIH. “How Sleep Works: Your Sleep/Wake Cycle.” Last updated March 24, 2022. Read September 20, 2026. NHLBI
Ohayon MM, Carskadon MA, Guilleminault C, Vitiello MV. “Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals: developing normative sleep values across the human lifespan.” Sleep. 2004. Read September 20, 2026 (abstract). PMID 15586779

This guide is for information and is not medical advice. Talk to a healthcare provider about medication, pregnancy, medical conditions and persistent sleep problems.