night owl

/ˈnaɪt aʊl//ˈnaɪt ˌaʊl/

From night + owl, from the fact that the bird is active at night.

1literallyAn owl (order Strigiformes) that is nocturnal.
2idiomaticOne who goes to bed late, or stays up late at night or in the early hours of the morning.
3(music) A musical instrument which imitates an owl's hoot, consisting of a receptacle partly filled with water and a mouthpiece that is blown into.