By evolutionary adaptation to the perpetual day-night changes in envi- ronmental conditions, eukaryotic organisms have acquired an endogen- ous programme. This mechanism exhibits the characteristics of a self- sustaining oscillation the period of which approximates that of the earth's rotation. For animals such a property was first clearly demon- strated by Maynard S. Johnson (1939) who recorded, in constant conditions, free-running activity rhythms of white-footed mice (Peromyscus /eucopus). Johnson concluded from his observations that "this animal has an exceptionally substantial and...