“Circadian rhythms are physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment.

Our biological clocks drive our circadian rhythms. These internal clocks are groupings of interacting molecules in cells throughout the body. A “master clock” in the brain coordinates all the body clocks so that they are in synch.”

via Cool facts about your biological clock | Human World | EarthSky.