1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): night owl
Fig. someone who stays up at night; someone who works at night. • My roommate is a night owl and usually reads until 5:00 a.m. • A few night owls stayed at the café all night long.
McGrawhill's American Idioms And Phrasal Verbs