"owl which flies at night," 1590s; applied since 1846 (American English) to persons who are up or out late at night. Cf. night-hawk, also French hirondelle de nuit "prostitute," literally "night-swallow."
A person who habitually stays up late and is active at night, as in You can call her after midnight, for she's a night owl. This colloquial term, originally used in the late 1500s for an owl that is active at night, was transferred to nocturnal human beings in the mid-1800s.