Biological clock, mechanism that controls the rhythm of various activities of plants and animals. Some activities, such as mating, migration, and hibernation, have a yearly cycle; others, chiefly reproductive functions (including human menstruation), follow the lunar month. But the majority have a period of roughly 24 hours, called the circadian rhythm. Although related to the day/night cycle, circadian rhythms are not directly controlled by it. Many organisms in unvarying environments will continue to show 24-hour rhythms, but the pattern can be changed and the clock “reset.” In December 1997 a human clock gene “rigui” was discovered, named after the ancient Chinese sundial.