Sleep and Sleep Disorder Fundamentals
Sleep regulation and architecture
Sleep timing and depth reflect two interacting systems: a homeostatic sleep drive that builds with wakefulness (adenosine accumulation) and a circadian pacemaker in the suprachiasmatic nucleus entrained by light and expressed through melatonin and core body temperature. Wake-promoting neurotransmitters include orexin/hypocretin, histamine, norepinephrine, serotonin, and acetylcholine; sleep-promoting pathways include GABAergic/galaninergic output from the ventrolateral preoptic area.
Sleep is scored in 30-second epochs and organized into cycles of roughly 90 minutes. N1 is the transition stage: theta activity, slow rolling eye movements, vertex sharp waves, loss of alpha. N2 is defined by K-complexes and sleep spindles. N3 requires slow waves (0.5–2 Hz, >75 µV) in at least 20% of the epoch. REM shows low-voltage mixed-frequency EEG, rapid eye movements, sawtooth waves, and the lowest chin EMG tone. Approximate adult proportions are N1 5%, N2 45–55%, N3 15–25%, and REM 20–25%; slow-wave sleep dominates early cycles, while REM periods lengthen toward morning.
ICSD-3 organizes disorders into six groups: insomnia, sleep-related breathing disorders, central disorders of hypersomnolence, circadian rhythm sleep-wake disorders, parasomnias, and sleep-related movement disorders.
Common question traps
N1 and REM share a low-voltage mixed-frequency EEG, so distinguish them using eye movements and chin tone. A single slow wave does not make N3. Question writers often imply REM fills the first cycle, or that poor sleep hygiene causes insomnia rather than contributing to it.
Rule of thumb: if the EEG is low-voltage mixed-frequency, check chin EMG and EOG - flaccid chin plus rapid eye movements means REM; preserved tone plus slow rolling eyes means N1.