Stages of Sleep: N1, N2, N3 and REM
Sleep is divided into non-rapid eye movement (NREM) sleep, which has three stages, and rapid eye movement (REM) sleep. The stages repeat in cycles of roughly 90 minutes through the night, with more deep N3 sleep early in the night and longer REM periods toward morning. Percentages below are typical rough figures for healthy young adults.
| Stage | Depth | Brain activity (EEG) | What happens in the body | Typical share of the night (adults) |
|---|---|---|---|---|
| N1 | Lightest sleep, transition from waking | Alpha rhythm gives way to slower theta activity; vertex waves | Slow rolling eye movements, muscle tone still present, easy to wake | About 5 percent |
| N2 | Light sleep | Sleep spindles and K-complexes | Heart rate and body temperature fall; the largest share of total sleep | About 45 to 55 percent |
| N3 | Deep sleep (slow wave sleep) | High amplitude slow delta waves | Hardest stage to wake from; growth hormone release; most common in the first half of the night | About 15 to 25 percent |
| REM | Active sleep, most vivid dreaming | Low voltage mixed frequency activity resembling wakefulness; sawtooth waves | Rapid eye movements, near paralysis of most skeletal muscles (atonia), irregular breathing and heart rate | About 20 to 25 percent |
Notes
- Stage names follow the AASM scoring manual, which merged the older stages 3 and 4 into N3 in 2007.
- N3 declines with age, and older adults have more N1 and more awakenings.
- Consumer wearables estimate stages from movement and heart rate; only polysomnography with EEG measures them directly.
Frequently asked questions
Which sleep stage is most important?
All of them play roles. Deep N3 sleep is linked with physical restoration and memory, and REM with emotional processing and memory. Getting enough total sleep is the best way to get enough of each.
Why do I dream more in the morning?
REM periods get longer in later cycles, so more of the last part of the night is REM sleep.
Is it bad if my tracker shows little deep sleep?
Not necessarily. Wearable stage estimates are much less accurate than a sleep study, and deep sleep normally decreases with age.
Sources
More reference tables
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