What are the normal stages of sleep, including non‑rapid eye movement (NREM) stages N1, N2, N3 and rapid eye movement (REM), and their typical duration and characteristics?

Normal Sleep Stage Architecture

Sleep consists of two distinct states—non-rapid eye movement (NREM) sleep with three stages (N1, N2, N3) and rapid eye movement (REM) sleep—that cycle approximately every 90 minutes throughout the night, with 4-6 cycles occurring during a typical sleep period. 1

NREM Sleep Stages

Stage N1 (Light Transitional Sleep)

• Represents the lightest stage of sleep and serves as the transitional phase from wakefulness to sleep 1
• Characterized by prominent and frequent slow eye movements, with arousal from this state being easiest among all sleep stages 2
• This is the most superficial sleep stage with the lowest arousal threshold 1

Stage N2 (Intermediate Sleep)

• Defined by higher arousal thresholds than N1 and represents the stage where most sleep time is spent 1
• Distinguished by the presence of sleep spindles and K-complexes on EEG, which are the defining electrophysiological features 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

Stage N3 (Deep/Slow Wave Sleep)

• Also termed "delta sleep" or "slow wave sleep," this stage is associated with the highest arousal threshold of all sleep stages 1
• Characterized by slow wave activity that must exceed a certain amplitude on EEG 1
• Has minimal eye movements and represents the deepest, most restorative sleep phase 2

REM Sleep Characteristics

• Occurs more frequently in the last half of the night, with arousal thresholds similar to N2 sleep 1
• Characterized by rapid eye movements, muscle atonia, and desynchronized EEG patterns 3
• Consists of tonic and phasic components, with the phasic component including clusters of rapid eye movements and muscle twitches 4
• Interscorer agreement for REM sleep identification is highest among all sleep stages at 78-94% 1

Sleep Cycle Duration and Distribution

• Sleep progresses in approximately 90-minute cycles (ranging from 90-110 minutes) alternating between NREM and REM sleep 1, 4
• Approximately 4-6 cycles emerge during the night, with REM episodes being prolonged toward morning 4
• About 70-80% of total sleep belongs to NREM sleep and 20-25% to REM sleep 4
• The distribution of sleep stages varies across the night, with deeper NREM sleep predominating in the first half and REM sleep increasing in the second half 1

Clinical Significance

• Understanding sleep stage architecture is essential for diagnosing sleep disorders through polysomnography, which remains the gold standard for evaluating sleep structure 1
• The presence and characteristics of eye movements, EEG patterns, and arousal thresholds allow clinicians to identify abnormalities in sleep architecture 1
• Eye movements are one of the key physiological parameters used by the American Academy of Sleep Medicine to differentiate between sleep stages 2

Age-Related Changes

• Normal aging is associated with a reduction in slow-wave sleep (N3), with the most marked changes occurring between ages 19-60 years 5
• Sleep in elderly individuals is characterized by increased number of sleep stage shifts, increased awakenings, and a shift toward more superficial sleep stages 4
• REM sleep duration remains relatively constant with aging 4