NREM

NREM, or non-rapid eye movement sleep, is the portion of sleep that encompasses three stages (N1, N2, N3) and does not involve rapid eye movements. It begins with light sleep (N1), where brain activity transitions from wakefulness to theta waves (approximately 4–7 Hz) and muscle tone remains reduced. N1 is followed by N2, marked by a further drop in arousal and distinctive features such as sleep spindles (short bursts of 12–16 Hz activity) and K-complexes on the electroencephalogram. N2 represents a stable, intermediate stage during which eye movements cease and the sleeper becomes less responsive to external stimuli. Deep sleep, or N3, is characterized by high-amplitude delta waves (0.5–4 Hz) and strong cortical synchronization; awakening from N3 is typically difficult and may involve sleep inertia. Throughout NREM, autonomic functions gradually slow and become more regular compared with waking.

Together with REM sleep, NREM accounts for the typical architecture of sleep across the night. In adults, NREM makes up about 75–80% of sleep, with REM occupying the remainder; the first half of the night features more deep sleep, while later cycles contain longer REM periods. Functions associated with NREM include memory consolidation, particularly for declarative memories, and restorative processes such as energy restoration and synaptic homeostasis. Disruptions of NREM can contribute to disorders of arousal, including sleepwalking and night terrors, and certain sleep disorders involve impaired NREM sleep. Aging, sleep deprivation, and pharmacologic factors can reduce the proportion of slow-wave sleep (N3) and alter the balance of NREM stages.