"owl which flies at night," 1593; applied since 1846 (Amer.Eng.) to persons who are up or out late at night. Similarly, night-hawk, used since 1611 of various birds; fig. sense of "one who seeks prey at night, one who stays up late" is from 1818. Cf. Fr. hirondelle de nuit "prostitute," lit. "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.