Circadian (circa 24 h) clocks confer a temporal structure on biological processes. Although they have been largely defined based on the surprising quality of a free-running rhythm with a frequency of about a day, in nature these timers serve to segregate various processes to different-presumably optimal-times of the 24 h day. Circadian clocks are cell-based, meaning that although the clock in an animal can be discerned through the timing of behavior, circadian rhythms are also apparent in each cell of the pacemaker in the brain, the suprachiasmatic nucleus. They are also ticking in skin cells, organ cells and cells that have been in tissue culture for decades.
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