To what does the term "circadian rhythm" refer?

A circadian rhythm is a 24-hour period affecting the physiology of all living creatures, including plants and animals. While some circadian rhythm functions are governed internally, others are subject to outside controls, including light and dark.

Biological clocks are the internal mechanisms controlling circadian rhythms in various organisms. These clocks are actually collections of molecules that interact in cells throughout a given organism's body. A group of nerve cells in the brain serve as a master clock, coordinating all biological clocks. These biological clocks regulate circadian rhythms and their resulting behaviors and bodily changes, such as sleep, hormone function and body temperature.