Sleep and Consciousness
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Direct answer
A night's sleep runs four to six cycles of about 90 minutes each: non-REM sleep occupies 75-80 per cent (stages N1, N2 and N3) and REM sleep 20-25 per cent, with the first REM period starting roughly 90 minutes after sleep onset and REM episodes lengthening toward morning. N3 slow-wave sleep — delta waves under 4 Hz, the deepest stage — is when growth hormone secretion peaks in the first 90 minutes and when sleepwalking and night terrors occur; REM sleep pairs a desynchronised, fast "paradoxical" EEG with complete postural muscle atonia (glycine and GABA inhibition of motor neurons), rapid eye movements, vivid dreams, and irregular heart rate and blood pressure. EEG bands organise the exam's questions: beta 13-30 Hz alert, alpha 8-13 Hz relaxed with eyes closed over the occipital region, theta 4-7 Hz drowsy, delta under 4 Hz deep sleep. Consciousness itself depends on the ascending reticular activating system in the brainstem: destructive lesions cause coma, and the suprachiasmatic nucleus of the hypothalamus runs the circadian clock that melatonin, secreted in darkness, follows.
What you must remember
- Cycle architecture: 90-minute cycles, 4-6 per night; NREM 75-80 per cent, REM 20-25 per cent; REM periods grow longer and N3 shrinks across successive cycles, so most slow-wave sleep sits in the first third of the night.
- Stage markers: N1 theta with rolling eye movements; N2 sleep spindles (12-14 Hz) and K complexes; N3 delta over 20-50 per cent of the tracing (20 per cent threshold for scoring N3); REM with sawtooth waves, atonia, rapid eye movements.
- EEG frequencies: beta above 13 Hz (alert, eyes open), alpha 8-13 Hz (relaxed, eyes closed, blocked by opening the eyes — Berger effect), theta 4-7, delta under 4.
- Physiology by stage: N3 carries growth hormone peaks, low heart rate, blood pressure and sympathetic tone; REM brings irregular autonomic activity with penile tumescence and the atonia that prevents acting out dreams.
- Parasomnia mapping: sleepwalking and night terrors arise from N3 in the first third of the night; nightmares are REM phenomena; REM sleep behaviour disorder is loss of REM atonia (linked to synucleinopathies).
- Consciousness anatomy: the ascending reticular activating system projects through the thalamus to the cortex; the suprachiasmatic nucleus above the optic chiasm is the master clock, entrained by light and driving pineal melatonin secretion in darkness.
- Drug effects: alcohol and benzodiazepines suppress REM (rebound insomnia and vivid dreams on withdrawal); benzodiazepines also reduce slow-wave sleep.
One night, walked through
Stage N1 arrives in minutes, theta waves replacing the waking beta of a mind still drifting. Ten minutes later, N2: spindles burst from thalamocortical loops at 12-14 Hz and K complexes dot the tracing. By half an hour, N3 delta sleep: pressure and heart rate at their nocturnal floor; the anterior pituitary releases its greatest growth hormone pulse, and a child may sit up glassy-eyed and sleepwalk. About 90 minutes in, the EEG accelerates and desynchronises, eyes dart under closed lids, and every postural muscle goes flaccid through glycine-mediated inhibition while the mind dreams. The night alternates, each pass trading N3 for more REM, so deep physical restoration sits early and emotional-memory processing late. The screening exam asks these markers in the same order.
The trap in stage identification
The trap is pairing the right phenomenon with the right stage. Nightmares versus night terrors is the classic: nightmares are REM dreams with recall, while night terrors are N3 arousals with screaming and no memory — and sleepwalking shares the N3, first-third-of-night address. Second trap: REM is deep sleep by arousal threshold yet light by EEG — "paradoxical" because the cortex looks awake while the body is paralysed, so candidates wrongly file it as light sleep. Third, alpha appears with eyes closed and vanishes the instant they open — the expected answer whenever an EEG description mentions eye opening. And consciousness is an ARAS function — a small pontine lesion can coma a patient with an intact cortex, an asymmetry the exam quotes as a one-liner.
Frequently asked questions
What percentage of sleep is REM in a normal adult?
About 20-25 per cent, concentrated in cycles of roughly 90 minutes with the longest REM periods in the early morning hours.
In which sleep stage is growth hormone secretion maximal?
Stage N3 slow-wave sleep, particularly in the first 90 minutes of the night — the reason deep sleep matters for growth and repair.
What defines stage N2 on an EEG?
Sleep spindles at 12-14 Hz and K complexes superimposed on a theta background, occupying about half of total sleep time.
Why do night terrors and sleepwalking occur in the first third of the night?
Both are N3 slow-wave phenomena, and N3 concentrates in the first third of the night before REM periods lengthen.
What causes the muscle atonia of REM sleep?
Glycine and GABA-mediated inhibition of spinal motor neurons, so dream content is not acted out; its loss produces REM sleep behaviour disorder.