Rhythms of differing lengths affect behavior. There are daily (circadian) rhythms, monthly rhythms, estrus cycles and yearly, or annual, rhythms that bring about differing responses. Circadian rhythms often govern the animal's daily activity and rest. Often these determine the way an animal functions in its niche, that is whether it is nocturnal (active at night), diurnal (active during the day), or crepuscular (active during twilight times).

The onset of hibernation, aestivation, migration or breeding is often under the influence of the yearly cycle. Estrus and the birth of young frequently occur at set times, as does egg laying. The zookeeper needs to be familiar with the species-specific cycles in order to provide for the changing needs of the animal throughout the year. These needs may involve a change in amount or kinds of food, adjustment in light periods, heat or humidity, the provision of nest boxes and nesting materials.

All eukaryotes and some prokaryotes (cyanobacteria) display changes in gene activity, biochemistry, physiology, and behavior that wax and wane through the cycle of days and nights. Examples: the level of the hormone melatonin that rises in your body during the night and falls during the day. Fruit flies (Drosophila) hatch in greatest numbers just at dawn.

Even when an organism is placed in constant conditions (e.g., continuous darkness), these rhythms persist. However, without environmental cues, they tend to be somewhat longer or somewhat shorter than 24 hours - giving rise to the name circadian rhythms (L. circa = about; dies = day).