Section 1 — Sleep Architecture: NREM, REM, and the Polysomnography Signatures

~1:00 – Section 1 — Sleep Architecture: NREM, REM, and…

Bottom line: normal sleep cycles every 90 minutes through NREM stages N1, N2, N3, and REM, with the American Academy of Sleep Medicine (AASM) staging system replacing the older Rechtschaffen and Kales four-stage NREM nomenclature; N2 is defined by sleep spindles and K-complexes, N3 by delta waves and the highest arousal threshold (most restorative, growth hormone secretion), and REM by paradoxical electroencephalographic (EEG) desynchronization with skeletal muscle atonia.

Normal sleep is organized into a cyclical pattern of distinct stages with characteristic electroencephalographic (EEG), electrooculographic (EOG), and electromyographic (EMG) signatures. The American Academy of Sleep Medicine (AASM) updated the staging in 2007, replacing the older Rechtschaffen and Kales system (NREM stages 1-4 plus REM) with N1, N2, N3, and REM; old stages 3 and 4 were combined into N3.

N1 is the lightest stage of sleep and the gateway from wakefulness. It accounts for 2-5% of total sleep time. The EEG shows low-voltage mixed-frequency activity with disappearance of the posterior dominant alpha rhythm; theta waves at 4-7 Hz predominate. Slow rolling eye movements may be present. Muscle tone is reduced but not absent. Patients in N1 are easily aroused and may not perceive that they were asleep.

N2 is the predominant stage of sleep, accounting for 45-55% of total sleep time, and it is defined by two heavily tested EEG features. Sleep spindles are brief bursts of rhythmic activity at 12-14 Hz generated by the thalamic reticular nucleus, believed to play a role in memory consolidation and in gating sensory input to prevent arousal. K-complexes are large-amplitude biphasic waveforms that occur spontaneously or in response to external stimuli; they are thought to function as a cortical arousal-suppression mechanism. If a question asks which sleep stage is characterized by sleep spindles and K-complexes, the answer is N2.

N3 is deep sleep, also called slow-wave sleep or delta sleep. It is defined by predominance of high-amplitude low-frequency delta waves (less than 2 Hz) occupying at least 20% of the epoch. N3 is the most restorative stage, associated with growth hormone secretion, tissue repair, immune function, and energy conservation. Arousal threshold is highest during N3, meaning patients are most difficult to awaken from this stage. N3 is most abundant in the first third of the night and decreases across later cycles. Slow-wave sleep decreases significantly with aging, contributing to the perception of lighter, less refreshing sleep in older adults. Growth hormone secretion is most concentrated during N3, particularly in the first sleep cycle, with implications for inpatient rehabilitation: fragmented sleep may impair growth hormone secretion, tissue repair, wound healing, and muscle anabolism.

REM sleep combines cortical activation with peripheral muscle paralysis. The EEG shows low-voltage mixed-frequency activity resembling wakefulness, earning REM the name paradoxical sleep. Rapid conjugate eye movements in bursts are the defining EOG feature. Skeletal muscle tone is at its lowest, with functional atonia of all voluntary muscles except the diaphragm and extraocular muscles. Atonia is mediated by active inhibition of alpha motor neurons via brainstem (sublaterodorsal nucleus) projections and prevents dream enactment. When this fails, the result is REM sleep behavior disorder (RBD).

Normal sleep cycles through these stages in a predictable pattern. A complete cycle from N1 through N2, N3, and REM lasts approximately 90 minutes. A typical night contains four to six cycles. The composition shifts across the night: the first cycles are dominated by deep slow-wave sleep with brief REM, while later cycles contain less deep sleep and progressively longer REM periods. Most vivid dreams occur in the early morning hours before waking.

Source: Polysomnography multi-channel recording. Eumetaxas, Wikimedia Commons (CC BY-SA 3.0). https://commons.wikimedia.org/wiki/File:Polysomnography.png

With aging, sleep architecture changes in predictable ways: total sleep time decreases, sleep efficiency (percentage of time in bed actually spent asleep) decreases, N3 slow-wave sleep decreases substantially (some elderly produce almost no delta sleep), sleep fragmentation increases, and the circadian rhythm shifts earlier (advanced sleep-wake phase). Pediatric to adult sleep needs decline gradually: newborns sleep 14-17 hours, infants 12-15, toddlers 11-14, school-age children 9-12, adolescents 8-10, and adults 7-9.