When Does REM Sleep Occur?
REM sleep occurs cyclically throughout the night in approximately 90-minute intervals alternating with NREM sleep, with the majority of REM sleep concentrated in the latter half of the night. 1
Timing Within the Sleep Period
- REM sleep predominates in the second half of the night, while slow-wave sleep (N3) is concentrated in the first half. 1
- Sleep progresses in cycles of approximately 90 minutes, alternating between NREM and REM stages throughout the night. 1
- In healthy adults with good sleep quality, four to six complete sleep cycles occur per night, each averaging 90 minutes in duration. 1
Circadian Influence on REM Distribution
- REM sleep demonstrates a strong circadian rhythm, with peak occurrence during the early morning hours corresponding to the rising phase of the body temperature cycle. 2, 3
- Approximately 74% of REM sleep time occurs during the ascending phase of the circadian body temperature rhythm, indicating phase reversal between REM sleep and autonomic functions. 2
- This circadian pattern persists even under experimental conditions such as sleep deprivation or altered sleep-wake schedules, demonstrating the robust nature of REM timing. 2
REM Sleep Architecture
- Each REM period follows a preceding NREM sleep segment, with REM episodes occurring at intervals determined by the amount of elapsed NREM sleep time. 4
- REM sleep is characterized by rapid eye movements and distinct EEG patterns that differ from NREM sleep, with arousal thresholds similar to stage N2 sleep. 1
- The proportion of total sleep time spent in REM remains relatively consistent at approximately one-quarter of prior NREM sleep time, regardless of total NREM duration. 4
Clinical Considerations
Common pitfall: Sleep interruptions by brief or prolonged awakenings can disrupt these normal cycles and fragment REM sleep distribution. 1, 5
- In older adults, REM sleep still follows the same approximately 90-minute cycle pattern, though total REM sleep amount decreases with age. 5
- Certain medications commonly used in elderly patients—including tricyclic antidepressants, monoamine oxidase inhibitors, and selective serotonin reuptake inhibitors—can suppress REM sleep and alter its normal timing. 5