A biological a clock is a self–sustained- internal timing mechanism that controls cyclic pattern, or rhytms, of a living organism. By providing temporal information , such as the time of day, month, or season, nearly all organism are adapted to event in both their internal or external environments. The product of this internal timing are physiological or behavioral even that, over time , vary in intensity.
Such biology time - dependent variability is expressed as a rhythm, or oscillation, with frequency equal to that of the underlying biological clock. Rhythms that occur more than once a day are called ultradian rhythm; one a day circadian; once a lunar day, lunar; and a once of year, circanual.
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