ABSTRACT

One of the most obvious adaptive features o f living organisms on earth is the ability of almost all species to change their behavior on a daily or 24-hr basis. In particular, many animals are active and awake only during certain times of the 24hr day, and they are inactive (resting and/or sleeping) at other times of the day. Such changes in the rest-activity/sleep-wake cycle are highly regimented within a given species, whereas the time and duration of rest and activity times vary greatly between species. Daily changes in life-style are, of course, correlated with the dramatic changes that take place in the physical environment due to the rotation of the earth on its axis. Although not as readily apparent as the behavioral changes, just about every aspect of the internal environment of the organism also undergoes pronounced fluctuations over the course of the 24-hr day. Claude Bernard’s concept of homeostasis, where the “milieu intérieur” remains constant in living organisms, has been modified over the last 30-40 years to take into account the fact that regular and predictable 24-hr changes in the internal milieu are a hallmark of most living organisms.