diurnal

Diurnal refers to anything related to the daytime or to a daily 24-hour cycle that includes the daylight period. The word comes from the Latin diurnus, meaning daily.

In biology, diurnal describes organisms that are active primarily during daylight hours. It is contrasted with nocturnal (active at night) and crepuscular (active during dawn and dusk). Diurnal species typically show activity patterns governed by circadian rhythms and responses to photoperiod. Humans, many birds, and many mammals such as primates are examples of diurnal organisms. Diurnal plants often open their flowers and perform photosynthesis during the day; examples include sunflowers and daisies.

In ecology and meteorology, diurnal variation refers to patterns that occur within a single day. This includes diurnal temperature variation, where daytime heating raises temperatures and nighttime cooling lowers them. Coastal regions may exhibit diurnal tides in some locations, characterized by a once-daily high and low tide, in contrast with semi-diurnal tides that occur about twice daily.

In astronomy, the term diurnal relates to phenomena that occur over the course of a day, such as diurnal motion—the apparent daily movement of stars and other celestial objects caused by Earth's rotation. The concept is used to distinguish daily patterns from longer-term celestial cycles.