How the body clock and sleep pressure influence sleepiness, sleep patterns, slow brain waves, and sleep spindles in people

A robust circadian rhythm of REM sleep peaks shortly after the core body temperature's lowest point.

• Sleep episodes of 9.33 hours occurred at all phases of the endogenous circadian cycle over 33-36 days in an environment free of time cues.
• An increase in REM sleep was observed to be sleep-dependent and interacted with the circadian rhythm, resulting in the highest REM sleep values coinciding with habitual wake-time.
• Slow-wave activity decreased while sleep spindle activity increased across all sleep episodes.
• Slow-wave activity in non-REM sleep showed low amplitude circadian modulation that did not align with the circadian rhythm of sleep propensity.
• Sleep spindle activity exhibited a distinct circadian rhythm, peaking at the start of the habitual sleep episode.
• The interaction between circadian and sleep-dependent components of sleep structure supports the consolidation of sleep and wakefulness.

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