What are the normal stages of sleep (non‑rapid eye movement and rapid eye movement) in healthy adults and how long does each stage typically last?

Normal Sleep Stages in Healthy Adults

Sleep consists of two distinct physiological states—non-rapid eye movement (NREM) and rapid eye movement (REM) sleep—that cycle approximately every 90 minutes throughout the night, with NREM subdivided into three stages (N1, N2, and N3) according to the American Academy of Sleep Medicine classification. 1, 2

NREM Sleep Architecture

Stage N1 (Light Sleep)

• N1 represents the lightest stage of sleep and serves as the transitional phase from wakefulness to sleep 2
• This stage has the lowest arousal threshold among NREM stages 1
• Typically comprises a small percentage of total sleep time in healthy adults 1

Stage N2 (Intermediate Sleep)

• Stage N2 is where most sleep time is spent and is characterized by higher arousal thresholds than N1 1, 2
• Distinguished by the presence of sleep spindles and K-complexes on EEG recordings 2
• Represents the predominant sleep stage across the night in healthy adults 1

Stage N3 (Deep/Slow Wave Sleep)

• Stage N3, also termed "delta sleep" or "slow wave sleep," is associated with the highest arousal threshold of all sleep stages 2, 3
• Characterized by slow wave activity that must exceed specific amplitude thresholds, preferentially measured in frontal EEG derivations 2, 3
• Occurs predominantly in the first half of the night 1
• In healthy adults, N3 sleep declines most dramatically between ages 19-60 years, with only modest reductions after age 60 3, 4

REM Sleep Characteristics

• REM sleep occurs more frequently in the last half of the night with arousal thresholds similar to N2 sleep 2
• Associated with rapid eye movements and characteristic EEG patterns distinct from NREM sleep 1
• Has the highest interscorer agreement (78-94%) among all sleep stages for identification 2

Sleep Cycle Duration and Distribution

• Sleep progresses in approximately 90-minute cycles alternating between NREM and REM sleep 1, 2
• In healthy individuals, good sleep quality includes about four to six cycles per night, with each cycle lasting an average of 90 minutes 1
• More stage N3 sleep occurs in the first half of the night, while more REM sleep takes place in the last half 1
• This sleep pattern can be interrupted by awakenings which may be extremely brief or of prolonged duration 1

Important Clinical Considerations

Age-Related Changes

• The most striking change in sleep patterns in older adults is repeated and frequent interruption of sleep by long periods of wakefulness 1
• Other age-dependent changes include decreased total sleep time, reduced sleep efficiency, and decreased slow wave and REM sleep 1
• When examining "optimally aging" individuals with controlled medical and psychiatric comorbidities, most age-dependent sleep changes occur in early and middle adulthood (years 19-60) 1

Common Pitfall: Scoring Methodology

• The AASM scoring rules systematically yield higher measured N3 percentages than older Rechtschaffen & Kales criteria because they prioritize slow-wave activity in frontal EEG derivations over central regions 3
• Transitioning from older to AASM scoring can artificially raise an individual's measured N3 percentage by 10-20% in serial assessments 3
• This underscores the need to account for scoring methodology when comparing studies or tracking individual patients over time 3