night owl

Short definition from Wiktionary

Noun

1. 1
   An owl (order Strigiformes) that is nocturnal.
2. 2
   One who goes to bed late, or stays up late at night or in the early hours of the morning.
3. 3
   A musical instrument which imitates an owl's hoot, consisting of a receptacle partly filled with water and a mouthpiece that is blown into.
4. 4
   Alternative form of night owl.