Stability, precision, and near-24-hour period of the human circadian pacemaker.
Level 3 - non-randomized controlled study
Non-randomized controlled physiological cohort study in humans
PubMed 10381883 · doi:10.1126/science.284.5423.2177
What was done
Researchers measured the endogenous circadian rhythms of melatonin, core body temperature, and cortisol in healthy young and older individuals living under strictly controlled lighting conditions designed to eliminate confounding effects of light on circadian period estimation.
What was found
The intrinsic period of the human circadian pacemaker averaged 24.18 hours across both young and older age groups with a tight distribution, contrasting with earlier literature reporting a median 25.2-hour activity rhythm (range 13 to 65 hours) and a ~25-hour temperature rhythm. The abstract does not provide exact participant counts, standard deviations, or separate numerical values by age group.
Why it matters
This study corrected decades of assumptions that the human biological clock runs on an intrinsic ~25-hour cycle that shortens with age, establishing that humans possess a precise, near-24-hour circadian period consistent with other species.
Limits
The abstract does not report the sample size, participant demographics, study duration, or specific light levels used. Findings in healthy individuals may not generalize to clinical populations with disrupted circadian rhythms.
Cited by
- supports The intrinsic human circadian biological clock is slightly longer than 24 hours in almost everyone.