For millennia it was thought that the daily fluctuations in sleep and activity of humans and other animals was governed by external cues, most importantly sunrise and sunset. But experiments similar to those shown in Figure 3.1 established that even in the absence of external cues, animals continue to exhibit daily oscillations in their sleep, activity, eating, and body temperature. These cycles prove that there must be some internal clock—a circadian clock (circa meaning approximately, and dian meaning day)—governing the daily rhythms of our lives.
