Sleep disorders

Sleep disorders are conditions that impair the ability to fall asleep, stay asleep, or maintain healthy sleep architecture, or that produce abnormal behaviors during sleep. Normal sleep cycles through non-REM stages (N1, N2, and deep N3 slow-wave sleep) and REM sleep — where most vivid dreaming occurs alongside muscle atonia — roughly every 90 minutes, under circadian control by the suprachiasmatic nucleus.

The major categories map onto that architecture. Insomnia is difficulty initiating or maintaining sleep despite adequate opportunity. Obstructive sleep apnea involves repeated airway collapse causing nighttime hypoxia and daytime sleepiness. Narcolepsy reflects loss of orexin (hypocretin) signaling and features sleep attacks, cataplexy, and REM intrusion phenomena such as hypnagogic hallucinations and sleep paralysis. Parasomnias split by sleep stage: sleepwalking and night terrors arise from deep N3 sleep, while nightmares and REM sleep behavior disorder arise from REM. Circadian rhythm disorders, such as shift-work disorder, misalign the internal clock with the required schedule.

Sleep disorders matter well beyond fatigue: untreated apnea contributes to hypertension and cardiovascular disease, chronic insomnia worsens mood and metabolic health, and REM sleep behavior disorder can precede neurodegenerative disease. Poor sleep also degrades memory consolidation, attention, and safety.

USMLE Step 1 tests sleep stages, EEG waveforms, and disorder-stage associations; AP Psychology covers sleep, dreaming, and sleep disorders within the biology of behavior; and the NCLEX addresses sleep as a component of adult health promotion across the lifespan.