Physiology chapters
Sleep: REM and NREM
The two types of sleep, the stages of NREM sleep, the EEG waves of wakefulness and sleep, the sleep cycle across the night, the brain areas that control sleep, and common sleep disorders.
- Sleep has two types: NREM (slow-wave) sleep, deep and restful, and REM (paradoxical) sleep, when dreaming, rapid eye movements and loss of muscle tone occur.
- The night runs in cycles of about 90 minutes; deep NREM dominates early in the night and REM grows longer towards morning.
- On EEG, awake relaxed = alpha, alert = beta, light sleep = theta with spindles and K complexes, deep sleep = delta; REM looks like the awake EEG.
On this page
Overview
Sleep is a natural, reversible state of unconsciousness from which a person can be woken by sensory or other stimuli. This is what separates it from coma.
There are two types of sleep, which alternate through the night:
- Non-REM (NREM) or slow-wave sleep: deep, restful sleep.
- REM (rapid eye movement) sleep: the stage of vivid dreaming.
EEG waves
The electroencephalogram (EEG) records the electrical activity of the cortex through the scalp.
| Wave | Frequency | When seen |
|---|---|---|
| Beta | More than 14 Hz | Alert, eyes open, mental activity; also REM sleep |
| Alpha | 8–13 Hz | Awake, relaxed, eyes closed; strongest over the occipital region |
| Theta | 4–7 Hz | Light sleep; normal in children; emotional stress in adults |
| Delta | Less than 4 Hz (high voltage) | Deep sleep; infants; serious brain disease |
Opening the eyes replaces alpha waves with low-voltage beta waves (alpha block).
As a person goes from alert to deep sleep, the EEG becomes slower and of higher voltage, because many cortical neurons fire in step (synchronised).
NREM sleep
NREM sleep makes up about 75–80% of adult sleep. It is divided into stages by the EEG.
| Stage | Depth | EEG |
|---|---|---|
| N1 | Lightest; drowsiness | Alpha fades; theta waves |
| N2 | Light sleep | Sleep spindles (short bursts of 12–14 Hz) and K complexes |
| N3 | Deep (slow-wave) sleep | Delta waves |
In NREM sleep:
- Heart rate, blood pressure, breathing and metabolic rate fall.
- Muscle tone is reduced but not lost.
- Dreams may occur but are usually not remembered.
- Growth hormone secretion peaks during deep slow-wave sleep.
Sleep spindles and K complexes are the hallmark of stage N2.
REM sleep
REM sleep makes up about 20–25% of adult sleep. Episodes recur about every 90 minutes and get longer towards morning.
In REM sleep:
- The EEG shows low-voltage, fast (beta-like) waves, like the waking brain.
- Rapid eye movements occur.
- Muscle tone is almost completely lost (atonia), except for the eye muscles and the diaphragm.
- Vivid dreams occur and are often remembered if the person wakes.
- Heart rate and breathing become irregular.
- Penile erection (and clitoral engorgement) occurs.
- It is harder to wake the person by sensory stimuli than in light NREM sleep, yet people often wake spontaneously from REM in the morning.
REM is called paradoxical sleep because the brain is very active (EEG like wakefulness) while the body is paralysed and the person is deeply asleep.
Despite the active EEG, the muscles are atonic in REM sleep. That is why we do not act out our dreams.
The sleep cycle
A night's sleep has 4 to 6 cycles of about 90 minutes.
- Each cycle goes N1 → N2 → N3 → N2 → REM.
- Early in the night, deep NREM (N3) periods are long.
- Later in the night, REM periods lengthen and N3 becomes short or absent.
Newborns spend about half their sleep in REM. The share of REM and deep sleep falls with age; older people have little stage N3.
Control of sleep and wakefulness
- Reticular activating system in the brainstem: keeps the cortex awake. Damage causes coma.
- Raphe nuclei (serotonin) in the brainstem and areas of the hypothalamus promote sleep.
- Orexin (hypocretin) neurons in the lateral hypothalamus keep us awake and stabilise wakefulness.
- Pontine neurons (using acetylcholine) switch on REM sleep and the muscle atonia that goes with it.
- Suprachiasmatic nucleus of the hypothalamus is the body clock (circadian rhythm). It is set by light through the retina and controls melatonin release from the pineal gland at night.
Why we sleep
The exact purpose is not fully known. Sleep appears important for brain maturation, learning and memory consolidation, clearing waste from the brain, and restoring energy. Lack of sleep impairs attention, judgment and mood.
Clinical relevance
Narcolepsy
How it happens: loss of orexin (hypocretin) neurons in the hypothalamus.
What you see: sudden daytime sleep attacks, cataplexy (sudden loss of muscle tone triggered by emotion), sleep paralysis and vivid dreams on falling asleep. Sleep may begin directly with REM.
Sleepwalking, night terrors and bedwetting
How it happens: these occur during deep NREM (stage N3) sleep, mostly in the first part of the night.
What you see: the child is hard to wake and does not remember the episode.
REM sleep behaviour disorder
How it happens: the normal muscle atonia of REM sleep is lost.
What you see: the person acts out dreams, sometimes violently. It can precede Parkinson's disease.
Obstructive sleep apnoea
How it happens: the upper airway collapses during sleep, especially when muscle tone falls.
What you see: loud snoring, pauses in breathing, poor sleep quality and daytime sleepiness.
Insomnia
Difficulty falling or staying asleep; very common, often linked to stress, anxiety or depression.
Questions ask for the EEG wave of each state, the stage with spindles and K complexes, the features of REM sleep and why it is paradoxical, the stage in which sleepwalking and night terrors occur, how sleep changes across the night and with age, and the cause of narcolepsy.
Quick revision
- Alpha 8–13 Hz (relaxed, eyes closed); beta more than 14 Hz; theta 4–7 Hz; delta less than 4 Hz.
- N2: spindles and K complexes. N3: delta waves, deepest sleep.
- REM: beta-like EEG, eye movements, muscle atonia, dreams; paradoxical sleep.
- Cycles of about 90 minutes; REM lengthens towards morning.
- Sleepwalking and night terrors: deep NREM.
- Narcolepsy: orexin loss; sleep begins with REM; cataplexy.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What is REM sleep?
A stage of sleep with rapid eye movements, vivid dreaming, loss of muscle tone and an EEG similar to wakefulness.
Which EEG waves are seen in deep sleep?
Delta waves, which are slow and of high voltage.
In which stage of sleep are sleep spindles seen?
Stage N2 of NREM sleep.
What causes narcolepsy?
Loss of orexin (hypocretin)-producing neurons in the hypothalamus.