ABSTRACT
Chronobiology deals with recurrent phenomena in living organisms. The so-called biological rhythms result from the evolutionary adaptation of organisms to regular changes in the environment. One of the most striking biological rhythms is the circadian rhythm of general activity (see Aschoff, this volume). Biological rhythms are, however, not limited to a 24-hour period. They range from a fraction of a second to several weeks, months or even years (migration of salmon or eels). They concern not only animals but also plants. They can also be found at the level of the organ, cell or cell component. Biological rhythms seem to be under the control of endogenous self-sustaining oscillating processes (the so-called biological clocks). They persist in constant environmental conditions where periodic stimulations or “Zeitgebers” are absent. Biological clocks can be considered as genetically built-in mechanisms: It has been shown, for example, that circadian activity persists in successive generations of mice raised in a constant environment.
