Intrinsic near-24-h pacemaker period determines limits of circadian entrainment to a weak synchronizer in humans

Endogenous circadian clocks are robust regulators of physiology and behavio r. Synchronization or entrainment of biological clocks to environmental tim e is adaptive and important for physiological homeostasis and for the prope r timing of species-specific behaviors. We studied subjects in the laborato ry for up to 55 days each to determine the ability to entrain the human clo ck to a weak circadian synchronizing stimulus [scheduled activity-rest cycl e in very dim (approximate to1.5 lux in the angle of gaze) light-dark cycle ] at three approximate to 24-h periods: 23.5, 24.0, and 24.6 h. These studi es allowed us to test two competing hypotheses as to whether the period of the human circadian pacemaker is near to or much longer than 24 h. We repor t here that imposition of a sleep-wake schedule with exposure to the equiva lent of candlelight during wakefulness and darkness during sleep is usually sufficient to maintain circadian entrainment to the 24-h day but not to a 23.5- or 24.6-h day. Our results demonstrate functionally that, in normally entrained sighted adults, the average intrinsic circadian period of the hu man biological clock is very close to 24 h. Either exposure to very dim lig ht and/or the scheduled sleep-wake cycle itself can entrain this near-24-h intrinsic period of the human circadian pacemaker to the 24-h day.