animal behavior characterized by activity during the day, with a period of sleeping, or other inactivity, at night
Animals that are active during the day are called diurnal. They hunt or feed during the day and sleep during the night. Nocturnal animals do it the other way round.
Scientists sometimes use the word crepuscular to describe nocturnal animals that are not active when it is very dark at night; they are mostly active near dawn, dusk, and sometimes when the moon makes a lot of light.