Sleep and Sleep Disorders

1. Overview & Epidemiology

Normal sleep architecture

Sleep alternates between two physiological states detected on polysomnography (EEG + electro-oculogram + EMG): NREM sleep and REM sleep.

NREM stages (modern AASM / DSM-5-TR nomenclature N1–N3):

REM sleep ("paradoxical sleep"): sawtooth/desynchronised low-amplitude EEG resembling wakefulness; skeletal-muscle atonia (except diaphragm and extra-ocular muscles); rapid eye movements; vivid dreaming; autonomic instability (variable HR/BP/RR). ~20–25% of the adult night.

Cycling: sleep latency ~15–20 min → descent N1→N2→N3 → first REM at ~90 min (normal REM latency ≈ 90 min). A cycle lasts ~90 min; 4–6 cycles/night. Across the night, N3 (SWS) shortens and REM periods lengthen — most SWS is in the first third, most REM in the last third. With ageing, both SWS and REM decline and awakenings increase.

Sleep changes in depression (high-yield)

Major depressive disorder produces a characteristic polysomnographic signature (Kaplan & Sadock):

• Reduced REM latency (REM appears soon after sleep onset) — degree of shortening correlates with depression severity.
• Decreased slow-wave (delta) sleep; shorter N3.
• REM shifted earlier in the night, with increased REM density (more eye movements).
• Early-morning (terminal) awakening and reduced total sleep — 80–85% of depressed patients have hyposomnia.

Epidemiology of the major sleep-wake disorders

• Insomnia disorder — most prevalent sleep disorder; ~10% meet full criteria, ~30–40% have occasional symptoms. Female > male (~1.5:1), prevalence rises with age. Becomes chronic in ~9–10% (PSY-4.170). Usual precipitant is everyday stress.
• OSA — most common breathing-related disorder; male > female; strongly tied to obesity/age.
• Narcolepsy — rare (~0.02–0.05%); onset adolescence/young adulthood; HLA-DQB1*06:02 association in type 1.
• NREM parasomnias — predominantly childhood; sleepwalking peaks 8–12 y; sleep terrors 1–6% of children, <1–2% of adults.
• RLS — 2–7% of adults; female > male; associated with iron deficiency, pregnancy, renal failure.

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