The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with nightowl
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.