Diurnality is a plant or animal behavior characterized by activity during the day, with a period of sleeping, or other inactivity, at night. The common adjective used for daytime activity is "diurnal". The timing of activity by an animal depends on a variety of environmental factors such as the temperature, the ability to gather food by sight, the risk of predation, and many others. Different behaviors may occur at different times of year; diurnality is a cycle of activity with a twenty-four-hour period while other cyclic activities are described as circadian rhythms. Animals active at dawn or dusk are described as crepuscular while those active at night are nocturnal. Some animals, such as those living underground, may be active at any time of day or night.