Nocturnal/Diurnal

Definition

A diurnal animal is one that is most active during the day, and a nocturnal animal is one that is most active at night.

Characteristics

General Considerations

Virtually all organisms undergo rhythms associated with the day-night cycle, but the patterns of these rhythms can vary considerably from one species to the next. The most striking differences have to do with the coupling between the rhythms and the day-night cycle. Specifically, in some animals activity is elevated during the day, while in others activity is highest during the night. The former are referred to as “diurnal” organisms and the latter as “nocturnal” ones. In both cases, an internal “circadian” clock generates endogenous rhythms that have a period of approximately 24 h and that are synchronized, or “entrained,” to a 24 h day by environmental cues such as the light-dark cycle.

One of the key adaptations that made the evolution of mammals possible was a change in this circadian time-keeping system from one...