NREMsömnen

NREM sleep, or non-rapid eye movement sleep, is a group of sleep stages that account for the majority of a typical night’s sleep, distinct from REM sleep. In some Scandinavian languages, the term NREMsömnen is used to refer to this portion of sleep.

Non-REM sleep is divided into three stages: N1, N2 and N3. N1 is a light transitional stage with slowed brain activity and occasional hypnic jerks. N2 is marked by the appearance of sleep spindles and K-complexes on the EEG and little or no eye movement. N3, or slow-wave sleep, is the deepest stage and features high-amplitude delta waves.

Sleep architecture and timing: Across a typical night, NREM accounts for about 75–80% of total sleep, with REM making up the remainder. SWS (N3) is more prominent early in the night, while REM sleep becomes more frequent in the later cycles.

Physiological characteristics and functions: During NREM, heart rate, breathing, and metabolic rate decline; muscles are relaxed, and body temperature falls. The brain shows alternating bursts of spindle activity as well as delta-dominated activity in N3. Functional roles include restoration, energy conservation, and memory consolidation, particularly for declarative memory, with glymphatic clearance of waste more active during deep sleep.

Clinical considerations: Some nocturnal parasomnias, such as sleepwalking and sleep terrors, occur during N3. Sleep-disordered breathing and sleep fragmentation can disrupt NREM architecture and contribute to daytime symptoms. Aging is associated with a reduction in slow-wave sleep and lighter overall NREM sleep.