What are the stages of sleep?
Sleep runs in four stages, three non-REM and one REM, repeating every 80 to 100 minutes. What happens in each, and how the mix shifts.
Key takeaways
Sleep has four stages, three non-REM stages (N1, N2, and N3) and rapid eye movement (REM) sleep.
The stages repeat in cycles of 80 to 100 minutes, and most nights hold four to six of them.
Deep sleep (stage 3) comes in longer stretches in the first half of the night, while REM takes up more of each cycle toward morning.
Stage 2 fills the most time. In a sleep-lab study of 100 healthy adults, it took up a median of 43% to 48% of the night.
Deep sleep and REM both shrink with age. In the same study, deep sleep made up about 21% of the night in people 30 and under and about 15% in people over 60.
How sleep divides into stages
Sleep isn’t one steady state. Researchers split it into two types, rapid eye movement (REM) sleep and non-REM sleep, and non-REM sleep has three stages of its own, according to the National Institute of Neurological Disorders and Stroke (NINDS). That’s four stages in all: N1, N2, N3, and REM.
The stages repeat in a loop. The National Heart, Lung, and Blood Institute (NHLBI) puts one full cycle at 80 to 100 minutes, with four to six cycles in a usual night and sometimes a brief waking between them. The balance between the stages changes from one cycle to the next, so the first hours of sleep look different from the last.
Stage 1 (N1) is the shift from waking to sleep
Stage 1 is the changeover from being awake to being asleep, and it usually lasts only several minutes. The heartbeat and breathing slow down. So do eye movements, and the muscles relax, with an occasional twitch. Brain waves begin to slow from their daytime pattern.
It doesn’t take up much of the night. In a polysomnography study of 100 healthy adults aged 19 to 77, stage 1 took up a median of 9% of the night in women and 11% in men.
Stage 2 (N2) is light sleep
Stage 2 is still light sleep, but it’s the point where NHLBI describes a person as asleep. Heartbeat and breathing slow, the muscles relax even further, body temperature drops, and eye movements stop. Brain waves keep slowing, with brief bursts of electrical activity breaking up the slower rhythm.
People spend more time in stage 2 than in any other stage. In the study of 100 adults, the median was 48% of the night for women and 43% for men.
Stage 3 (N3) is deep sleep
Stage 3 is the deep sleep that NINDS says a person needs to feel refreshed in the morning. It’s also called slow-wave sleep, after the slow, large brain waves that show up on the recording. Heartbeat and breathing fall to their lowest levels of the night, and someone in this stage can be hard to wake.
Stage 3 comes in longer stretches during the first half of the night. Older scoring rules split this deep sleep into two stages, 3 and 4. The American Academy of Sleep Medicine’s current rules group them together as N3.
REM sleep is when most dreams happen
The first REM period starts about 90 minutes after falling asleep. Behind closed lids, the eyes dart from side to side. Brain activity moves closer to its waking pattern, breathing turns faster and irregular, and heart rate and blood pressure climb to near waking levels.
The limbs go limp, though. Arm and leg muscles are temporarily paralyzed during REM, which is why people don’t act out their dreams. The body also regulates its own temperature poorly in this stage, and NHLBI notes that people get less REM sleep in colder temperatures.
How the stages spread across a night
Each cycle passes through the stages, but the proportions change as the night goes on. The early cycles carry long stretches of deep sleep. Later cycles carry less deep sleep and more REM, so the last hours before waking hold the most REM.
Stage | Also called | What happens | When it’s most common |
|---|---|---|---|
N1 | Light sleep | Heartbeat, breathing, and eye movements slow, and muscles relax | At the start of sleep, for several minutes |
N2 | Light sleep | Body temperature drops, eye movements stop, and brain waves slow with brief bursts | Throughout the night, more than any other stage |
N3 | Deep sleep, slow-wave sleep | Heartbeat and breathing reach their lowest, and waking can be hard | Longer stretches in the first half of the night |
REM | Rapid eye movement sleep | Eyes move quickly, brain waves look closer to waking, and limbs are temporarily paralyzed | More of it later in the night |
How the stages change with age
The share of each stage shifts over a lifetime. Newborns spend more of their sleep in REM. Slow-wave sleep peaks in early childhood, drops sharply in the teenage years, and keeps declining through adulthood, according to NHLBI, and some older people may have none at all.
The study of 100 adults showed the same pattern within adulthood. Deep sleep made up a median of 20.7% of the night in people 30 and under and 14.9% in people over 60. REM fell from 15.5% to 10.3% across the same age groups. Those figures are from a single night in the lab. That’s a limitation the authors flag, since sleep in an unfamiliar lab tends to run lighter on the first night. A second sleep-lab study of 206 healthy adults found that sleep got shorter and lighter with age, with more time in stage 1, and that women had more deep sleep than men. It also found wide variation between healthy people of the same age, so a single person’s numbers can sit well outside these medians.
How sleep stages are measured
Sleep stages are identified with a polysomnogram, an overnight sleep study usually done in a sleep lab or sleep center. It records brain waves, eye and limb movements, breathing, oxygen levels, and heart rate through the night, and trained scorers use the brain and eye recordings to label each stretch of sleep by stage.
The labels come from a published scoring manual, and its rules have changed. Older articles and textbooks often list five stages (stages 1 to 4 plus REM) because they follow the earlier rules, which treated deep sleep as two separate stages.
Frequently asked questions
How long is one sleep cycle?
One cycle through the sleep stages takes about 80 to 100 minutes. Most nights include four to six cycles, and it’s possible to wake briefly between them.
Which stage of sleep is deep sleep?
Stage 3, also called N3 or slow-wave sleep, is deep sleep. It comes in longer stretches in the first half of the night, and it can be hard to wake someone who’s in it.
Do you only dream during REM sleep?
No. Most dreaming happens during REM sleep, and REM dreams tend to be the most vivid, but dreams can occur in every stage. People spend about two hours a night dreaming, according to NINDS, and may not remember most of it.
Why do some sources say there are five stages of sleep?
Older scoring rules divided deep sleep into stage 3 and stage 4. The current rules from the American Academy of Sleep Medicine combine them into one stage, N3, which leaves four stages in total.
ABOUT THIS ARTICLE
This article explains research on sleep in general and isn’t medical advice. If you have symptoms that worry you, or sleep trouble that lasts more than a few weeks, talk to a doctor.