Functional Genomics of Sleep and Circadian Rhythm Invited Review: A neural clockwork for encoding circadian time

neural clockwork for encoding circadian time. J Appl Physiol 92: 401‐408, 2002; 10.1152/japplphysiol.00836.2001.—Many daily biological rhythms are governed by an innate timekeeping mechanism or clock. Endogenous, temperature-compensated circadian clocks have been localized to discrete sites within the nervous systems of a number of organisms. In mammals, the master circadian pacemaker is the bilaterally paired suprachiasmatic nucleus (SCN) in the anterior hypothalamus. The SCN is composed of multiple single cell oscillators that must synchronize to each other and the environmental light schedule. Other tissues, including those outside the nervous system, have also been shown to express autonomous circadian periodicities. This review examines 1) how intracellular regulatory molecules function in the oscillatory mechanism and in its entrainment to environmental cycles; 2) how individual SCN cells interact to create an integrated tissue pacemaker with coherent metabolic, electrical, and secretory rhythms; and 3) how such clock outputs are converted into temporal programs for the whole organism. suprachiasmatic nucleus; period; oscillator; pacemaker; photoperiod THE EARTH’S DAILY ROTATION about its axis has imposed potent selective pressures on organisms. The fundamental adaptation to the environmental day-night cycle is an endogenous 24-h clock that regulates biological processes in the temporal domain. This clock coordinates physiological events around local (geophysical) time, optimizing the economy of biological systems and allowing for a predictive, rather than purely reactive, homeostatic control. Circadian clocks contribute to the regulation of sleep and reproductive rhythms, seasonal behaviors, and celestial navigation. The practical importance of human circadian rhythmicity, as well as its consequences for health and disease, is now being realized.