Circadian Systems: Entrainment

The circadian rhythmicity of eukaroytic organisms is dictated by an innate program that specifies the time course through the day of many aspects of metabolism and behavior. The programmed sequence of events in each cycle of the rhythm has been evolved to parallel the sequence of predictable change (physical and biological) in the course of the day-outside: it constitutes an appropriate day-within. It is a characteristic, almost defining, feature of these circadian programs that their time course is stabilized with almost clocklike precision to parallel the stable time course of the environmental day. There is equally clear functional significance to the program’s being driven by a self-sustaining oscillator; thus, the program is subject to entrainment by one or more of the external cycles whose period it closely approximates. It is this entrainability that provides for proper phasing of the program to the sequence of external changes that it has been evolved to cope with and exploit.