ABSTRACT
There are many different rhythms in human biology and behavior. The periods of these rhythms range from less than 1 second to more than 1 year. At least two biological rhythms have a special relationship to external rhythms in the natural environment. The intrinsic periods of these rhythms correspond to the solar day (circadian rhythm) and the solar year (circannual rhythm). Because these circarhythms (Aschoff, 1979) cause certain changes within the organism to anticipate and be coordinated with recurring changes in the environment, it is likely that they comprise a special form of adaptation to the environment. The circarhythms that arise within the organism not only mirror the rhythmic changes in the environment, but also are controlled by them. Periodic stimuli in the environment are processed by the organism in such a way that the circarhythms’ timing is rather precisely synchronized in a particular phase relationship with the environmental rhythms. Thus circarhythm generators are like slave oscillators: They produce their own rhythm but their frequency and phase are controlled by another oscillator (i.e., the environmental rhythm) to which they are coupled.
