Eukaryotes, living things whose cells contain membrane-enclosed nucleus and organelles, have internal clocks. Their clocks keep track of time and regulate cyclical processes known as biological rhythms. Biological rhythms are affected by factors in the environment, but basically they are genetically programmed into cells. Some rhythms, such as breathing and heartbeat, are short. Others, like the menstrual cycle and leaf-loss in trees, are long in duration. Circadian rhythms, like the sleep/wake cycle, involve intervals of about twenty-four hours. The word circadian comes from "circa" meaning about, and "dies" meaning day. Research shows that circadian rhythms are genetically acquired. The circadian cycle keeps going on indefinitely in an environment that has no external clues about time. Although biologists agree that most organisms have an innate clock mechanism in their cells, they do not know the exact nature of their timekeeper.

Biological rhythms occur in protists, fungi...