Three non-REM stages and REM sleep, cycling about every 90 minutes
Mnemonic
“Non-REM in three, then REM”, cycling roughly every 90 minutes:
- N1 - light sleep, theta waves, hypnic jerks
- N2 - sleep spindles and K complexes. The largest proportion of the night
- N3 - slow wave delta sleep. The deepest, and where sleepwalking, night terrors and bedwetting occur
- REM - rapid eye movements, dreaming, muscle atonia, and an EEG resembling wakefulness
“Slow wave sleep is for the body, REM is for the brain.”
Slow wave sleep predominates in the first half of the night and REM in the second, lengthening with each cycle, which is why dreams are recalled on waking and why alcohol, which suppresses REM early, causes vivid dreams and fragmented sleep later.
REM atonia is what fails in REM sleep behaviour disorder, an early marker of Parkinson’s disease and Lewy body dementia, and what is lost inappropriately in cataplexy and sleep paralysis in narcolepsy.
Expansion
Non-REM
- N1: light sleep, theta waves, hypnic jerks
- N2: sleep spindles and K complexes; the largest proportion of the night
- N3: slow wave (delta) sleep; deepest, hardest to rouse, dominant in the first half of the night. Growth hormone secretion peaks; night terrors and sleepwalking occur here
REM sleep
- Low voltage, mixed frequency EEG resembling wakefulness (hence paradoxical sleep)
- Rapid eye movements, dreaming, and muscle atonia except the diaphragm and extraocular muscles
- Irregular pulse, respiration and blood pressure
- Increases in duration through the night
Cycles last about 90 minutes, with about 4 to 6 per night.
Clinical corollaries: the REM atonia explains why obstructive sleep apnoea and neuromuscular hypoventilation are worst then; REM sleep behaviour disorder, where atonia fails, is strongly associated with later Parkinson disease; and alcohol, benzodiazepines and many antidepressants suppress REM, with rebound on withdrawal.