ABSTRACT
The lives of living things are characterized by changes in behavior that occur regularly over a 24-h cycle. As examples, consider the sleep-wake cycle present in all animals and the fact that many plants raise and lower their leaves from day to night. These rhythms, and the many others that occur, are called circarhythms. The physiology that controls such circarhythms is called the circadian system, from the Latin, circa, for “about,” and dies, for “day,” about a day. Circadian systems exist in a wide range of life, from unicellular organisms to humans, in insects, plants, fish, birds, and mammals. Further, there are cyclic patterns that occur over the seasons, called circannual rhythms, such as the seasonal breeding of mammals and seed germination in plants. These are believed to be controlled by the gradual change in light/dark ratio that occurs over the seasons, as signaled by the circadian system.
