Sleep Stages: NREM and REM Sleep Architecture
Sleep consists of two distinct physiological states—non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep—with NREM subdivided into three stages (N1, N2, and N3) according to the American Academy of Sleep Medicine classification. 1
NREM Sleep Stages
Stage N1 (Light Sleep)
- Represents the lightest stage of sleep and serves as the transitional phase from wakefulness to sleep 1
- Characterized by slow eye movements that are most prominent and frequent during this stage 2
- Has the lowest arousal threshold, making awakening easiest from this stage 2
- Most likely to be confused with Stage N2 during sleep scoring 1
Stage N2 (Intermediate Sleep)
- Defined by higher arousal thresholds and represents the stage where most sleep time is spent 1
- Distinguished by the presence of sleep spindles and K-complexes on EEG 1
- Slow eye movements can still occur but are less frequent than in N1 2
- Arousal thresholds during N2 are similar to those during REM sleep 2
- Sleep spindles and vertex waves are best visualized over central EEG derivations 1
Stage N3 (Deep Sleep/Slow Wave Sleep)
- Previously classified as stages 3 and 4 combined, now consolidated into a single N3 stage by the AASM 1
- Also termed "delta sleep" or "slow wave sleep" based on characteristic EEG profiles 1
- Associated with the highest arousal threshold of all sleep stages 1, 2
- Characterized by slow wave activity that must exceed 75 μV in amplitude and is preferentially scored in frontal EEG derivations 3
- Occurs predominantly in the first half of the night 1
- Has minimal eye movements 2
- Decreases dramatically with normal aging, with the most marked changes occurring between ages 19-60 years 4, 3
REM Sleep
- Characterized by rapid eye movements, muscle atonia, and desynchronized EEG activity 5
- Occurs more frequently in the last half of the night 1
- Has arousal thresholds similar to N2 sleep 2
- Interscorer agreement for REM sleep identification is highest among all sleep stages (78-94%) 1
Sleep Cycle Architecture
- Sleep progresses in approximately 90-minute cycles alternating between NREM and REM sleep 1
- A typical night includes 4-5 NREM-REM cycles 6
- The distribution of sleep stages varies across the night, with more N3 sleep in the first half and more REM sleep in the second half 1
- Sleep can be interrupted by awakenings of varying duration 1
Clinical Significance
Understanding sleep stage architecture is essential for diagnosing sleep disorders through polysomnography, which remains the gold standard for evaluating sleep structure 4, 3. The presence and characteristics of eye movements, EEG patterns, and arousal thresholds across different stages allow clinicians to identify abnormalities in sleep architecture that may indicate underlying pathology 2.