Diurnality is a form of 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". Animals active at dawn or dusk are crepuscular, those active at night are nocturnal, and animals active at sporadic times during both night and day are cathemeral.
Many animal species are diurnal, including many mammals, insects, reptiles and birds. In some animals, especially insects, external patterns of the environment control the activity (exogenous rhythms, as opposed to patterns inherent in the habitat). Diurnality is descriptive; it refers to an observed 24-hour pattern, as opposed to ~24-hour circadian rhythms which are self-sustaining within the organism.