Biological clocks drive behavioral cycles or rhythms that repeat at regular intervals. A 24-hour cycle is called a circadian rhythm, which comes from the Latin meaning "about a day." The most obvious example is our sleep-wake cycle, which even operates in the absence of environmental cues like daylight. In other words, a person will settle into a roughly 24-hour cycle of activity and sleep even in a darkened cave. Shorter rhythms, such as the rhythm of a beating heart, are termed "ultradian." Cycles longer than 24 hours are called "infradian," and if they last a year, such as migration and hibernation, they are called "circannual" rhythms.

Generating these biological rhythms internally, rather than depending completely on environmental cues, helps organisms anticipate important changes in the environment before they occur, thus providing the organisms with sufficient time to prepare.