ABSTRACT
Terrestrial life evolved with a sidereal period of Earth’s revolution about the sun close to 365 days and an axial rotation near 24 hr. It is thus not surprising that nearly all organisms studied, including prokaryotes and eukaryotes, possess a(n) internal biological clock(s) with a near-24-hr periodicity. These internal clock mechanisms or endogenous pacemakers determine the appropriate environmental timing for species specific biological and behavioral events (e.g., sleep, endocrine function, temperature regulation). It is generally accepted that the periodic lightdark cycle, associated with the earth’s daily axial rotation, is the dominant envi ronmental synchronizer used by nearly all organisms to entrain to the 24-hr geo physical day. Entrainment is a fundamental property of endogenous oscillators by which the period of the pacemaker is synchronized to the period of the entraining stimuli. In the absence of periodic synchronizing agents, endogenous pacemakers oscillate at their own intrinsic period. Controlled experimental studies have shown the endogenous pacemaker in humans to cycle with an average period that is somewhat longer than 24 hr (1-4). Stable entrainment of the endogenous pace maker to the light-dark cycle is achieved when the resetting response to light
exposure offsets the drift due to the longer-than-24-hr intrinsic period of the human circadian pacemaker.
