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People frequently talk about body clocks, a term that refers to the patterns of energy and exhaustion, functioning and resting, and wakefulness and sleep that characterize everyday life. In fact, the concept of the body clock, or circadian rhythm, is part of a larger picture of biological cycles, such as menstruation in mammalian females. Such cycles, which assume a variety of forms in a wide range of organisms, are known as biological rhythms....
