Sleep stages and their EEG

Concept

A night of sleep is a series of 90-minute cycles of light sleep, deep sleep and dream (REM) sleep, each with its own brain waves.

Updated Nov 5, 2026

Key points

Awake and alert: beta. Awake, relaxed, eyes closed: alpha. N1: theta and vertex waves. N2: sleep spindles and K complexes. N3: delta (slow waves). REM: sawtooth waves on a fast, low-voltage EEG.
N2 takes the largest share of the night (about half); REM about a fifth to a quarter; N3 about a sixth in young adults.
REM: rapid eye movements, near-total muscle atonia, vivid dreams, variable pulse and breathing, penile and clitoral tumescence; EEG similar to waking (paradoxical sleep).
N3 parasomnias: sleepwalking, sleep terrors and confusional arousals, early in the night, with little recall. REM parasomnias: nightmares (recalled) and REM sleep behavior disorder (acting out dreams).
The first REM period normally starts about 90 minutes after sleep onset; REM at sleep onset suggests narcolepsy, and short REM latency is also seen in depression.

What it is

Sleep is divided into non-REM sleep (N1, N2 and N3) and REM sleep, recognized on polysomnography by the EEG, the eye movements (electro-oculogram) and the chin muscle tone (electromyogram), scored in 30-second epochs. The stages follow each other in cycles of about 90 minutes, four to six times a night: deep N3 sleep fills the first cycles, and REM periods grow longer toward morning.

Key facts

Polysomnography records at least EEG, eye movements and chin muscle tone to stage sleep; respiratory flow, oxygen saturation and leg movements are added to diagnose disorders.
Arousal threshold rises from N1 to N3: a sleeper in N3 is hard to wake and often confused for a few minutes.
REM atonia spares the diaphragm and the eye muscles; it is generated in the pons.
Sleep deprivation is followed by rebound: N3 is recovered first, REM afterwards.

Sleep stages in a healthy young adult

Stage	EEG	Eyes and muscles	Share of the night	Behavior and clinical clues
Awake, alert	Beta: fast, low amplitude	Blinks, voluntary movements, high tone	—	Attention, eyes open
Awake, relaxed	Alpha, over the occipital region with eyes closed	Tone present	—	Disappears on opening the eyes
N1	Theta; vertex sharp waves	Slow rolling eye movements; tone slightly lower	About 5 %	Drowsiness, hypnic jerks; when woken, often denies having slept
N2	Sleep spindles and K complexes on a theta background	No eye movements; tone lower	About 45–55 %	True sleep; spindles are linked to memory consolidation
N3	Delta: slow, high-amplitude waves over at least a fifth of the epoch	No eye movements; low tone	About 15–20 %	Hardest to wake; growth hormone peak; sleepwalking, sleep terrors, enuresis
REM	Fast, low-voltage, sawtooth waves	Rapid eye movements; atonia (lowest chin tone)	About 20–25 %	Dreams, nightmares; REM sleep behavior disorder when atonia fails

Clinical notes

A child who screams, sits up and cannot be comforted two hours after falling asleep, with no memory in the morning, fits a sleep terror (N3); a frightening dream recalled in detail at dawn is a nightmare (REM).
An older man who punches and shouts while dreaming may have REM sleep behavior disorder, which can precede Parkinson disease and Lewy body dementia by years.
Narcolepsy brings REM features into waking: cataplexy (atonia triggered by emotion), sleep paralysis and hypnagogic hallucinations.

Test yourself

Think of the answer, then tap the question to see it.

Which EEG features define stage N2 sleep?

Sleep spindles (brief bursts of fast waves) and K complexes (large biphasic waves) on a theta background.

In which stage do sleepwalking and sleep terrors occur, and in which part of the night?

In N3 (slow-wave sleep), mostly in the first third of the night, when N3 is most abundant.

Why is REM sleep called paradoxical sleep?

Because the EEG is fast and low-voltage like waking and the brain is very active, while the skeletal muscles are almost completely atonic.

What EEG wave is seen in a relaxed, awake adult with eyes closed?

Alpha rhythm, most prominent over the occipital region; it disappears when the eyes open.

What does a REM period appearing within minutes of sleep onset suggest?

Narcolepsy; a shortened REM latency is also seen in major depression and after REM deprivation.

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Educational content on behavior and the clinical relationship; not a diagnostic or treatment guide and no substitute for assessment by a health professional. If you or someone near you is at risk, seek help now from an emergency service or a mental-health helpline.

Sources: How Sleep Works - Sleep Phases and Stages (National Heart, Lung, and Blood Institute (NHLBI), 2022); Approach to the Patient With a Sleep or Wakefulness Disorder - Neurology - MSD Manual Professional Edition (MSD Manual Professional Edition, 2026).

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