Nihon rinsho. Japanese journal of clinical medicineSep 23, 2009

Daily changes in the timing of the human sleep-wake cycle

Circadian Biology Weekly Brief ↗PubMed ↗

Updated Apr 2, 2026

Abstract

Bright light is a dominant cue for human circadian systems, with a circadian rhythm period exceeding 24 hours.

The human circadian system features at least two distinct oscillators with different periods.
One oscillator has a circadian period of about 25 hours, influencing core body temperature and melatonin rhythms.
Another oscillator operates with a period longer than 30 hours, affecting the sleep-wake cycle.
Internal desynchronization occurs when these oscillators free-run with different periods.
There is evidence of non-photic entrainment, where the sleep-wake cycle aligns with a strict social schedule.

Simplified

In humans, studies using temporal isolation unit have indicated that (1) bright light is a dominant zeitgeber in human circadian systems and (2) period of endogenous circadian rhythms is longer than 24 h. The human circadian systems have at least two unique features. First is an internal desynchronization in which period of core body temperature rhythm and sleep-wake cycle free-run with different periods. Internal desynchronization indicates that the human circadian system consists of two different oscillators. One has a circadian period of about 25 h and drives the rhythms in rectal temperature and melatonin, and the other has a period longer than 30 h and drives the sleep-wake cycle. Second feature is non-photic entrainment of sleep-wake cycle to a strict social schedule (so called partial entrainment). In this review, we summarize and update the two oscillator model based on recent evidence obtained by sleep-wake schedule and physical exercise experiments under temporal isolation facility.

Daily changes in the timing of the human sleep-wake cycle

Abstract

You found one interesting study. We’ll send the next 7.

What Lands in Your Inbox Each Week:

Recent issues from the circadian biology brief