[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Behavioral science](https://durgical.com/en/behavioral-science/) › [Normal sleep](https://durgical.com/en/behavioral-science/areas/sleep/) › Sleep stages and their EEG

# 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](https://durgical.com/en/conditions/nocturnal-enuresis/) | 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](https://durgical.com/en/conditions/rem-sleep-behaviour-disorder/) | 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.

## Memory aid

From the desk to the deep: Beta at work, Alpha with eyes closed, Theta while nodding off, Spindles and K complexes to stay asleep, Delta in the deep, and the Sawtooth of dreams.

## 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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## Elsewhere in DurgiPedia

- [Wakefulness, sleep, the EEG and seizures](https://durgical.com/en/physiology/wakefulness-sleep-the-eeg-and-seizures/)
- [Growth hormone and IGF-1](https://durgical.com/en/physiology/growth-hormone-and-igf-1/)

## Connections

## Related 3

- [Normal sleep across the life span](https://durgical.com/en/behavioral-science/normal-sleep-across-the-life-span/)
- [How sleep is regulated: sleep pressure, the circadian clock and melatonin](https://durgical.com/en/behavioral-science/how-sleep-is-regulated-sleep-pressure-the-circadian-clock-and-melatonin/)
- [Sleep hygiene and the behavioral treatment of insomnia](https://durgical.com/en/behavioral-science/sleep-hygiene-and-the-behavioral-treatment-of-insomnia/)

## Related conditions 5

- [Narcolepsy](https://durgical.com/en/conditions/narcolepsy/)
- [REM sleep behaviour disorder (RBD)](https://durgical.com/en/conditions/rem-sleep-behaviour-disorder/)
- [Nocturnal enuresis](https://durgical.com/en/conditions/nocturnal-enuresis/)
- [Obstructive sleep apnoea](https://durgical.com/en/conditions/obstructive-sleep-apnoea/)
- [Major depressive disorder](https://durgical.com/en/conditions/major-depressive-disorder/)

[← All of behavioral science](https://durgical.com/en/behavioral-science/)

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](https://www.nhlbi.nih.gov/health/sleep/stages-of-sleep) (National Heart, Lung, and Blood Institute (NHLBI), 2022); [Approach to the Patient With a Sleep or Wakefulness Disorder - Neurology - MSD Manual Professional Edition](https://www.msdmanuals.com/professional/neurologic-disorders/sleep-and-wakefulness-disorders/approach-to-the-patient-with-a-sleep-or-wakefulness-disorder) (MSD Manual Professional Edition, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/behavioral-science/sleep-stages-and-their-eeg/