Why do REM periods become longer later in the night?

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In the human sleep cycle, the duration of REM sleep periods changes significantly as the night progresses. While early REM periods in adults are relatively short—lasting approximately 10 minutes—their duration increases throughout the night, eventually reaching up to about 60 minutes in later cycles.1

This shift in duration is a characteristic feature of the ultradian cycles that organize sleep stages within a single nocturnal period. While the specific physiological reasons for this lengthening remain a subject of ongoing study, it is known that the distribution of sleep stages is not uniform; for instance, deep NREM sleep (slow-wave sleep) is concentrated in the early portion of the night, whereas REM sleep becomes more prominent as the night progresses toward morning.2