Activity Patterns and the Biological Clock in Mammals

Abstract

One of the most consistent features of the physical and social environment is change. Many changes occur in an unpredictable manner, whereas others occur in precise rhythmic patterns. It is thought that internal clocks evolved as a way of adapting behaviour and physiology to these regularly occurring changes in the environment. Although biological clocks can be set to a number of different cycle lengths (see Aschoff 1981), clocks that exhibit daily cycles are prevalent in mammals. Internal clocks that time daily rhythms are called ‘circadian clocks’. Their name is derived from the Latin ‘circa’ meaning about or around and ‘dies’ or ‘dian’ meaning day. It sounds as if these clocks might be fairly imprecise because their name suggests that they time cycles that only approximate a day. In fact, circadian clocks are extraordinarily precise and are found frequently to be inaccurate by less than a minute in each cycle (Pittendrigh and Daan 1976a). These clocks received their name because they measure out a genetically determined daily cycle that deviates slightly, but significantly from 24 h. In animals, the ‘period’ (i.e., time required to complete one full cycle) of circadian cycles usually ranges between 23–26 h. Although the period of circadian clocks varies among species and individuals of the same species, within an individual the period remains quite consistent.