night owl

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who habitually stays up and is active late into the night.

from The Century Dictionary.

• noun An owl of notably or exclusively nocturnal habits. All owls are nocturnal, but some less so than others, and night-owl is used in contrast to day-owl.

from the GNU version of the Collaborative International Dictionary of English.

• noun A person who likes to be active late at night.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic One who stays up late at night or goes to bed late.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who likes to be active late at night