A person who likes to be active late at night WordNet 3.0 & Open English WordNet

A night owl, evening person or simply owl, is a person who tends or prefers to be active late at night and into the early morning, and to sleep and wake up later than is considered normal; night owls often work or engage in recreational activities late into the night, and sleep until relatively late in the day. Wikipedia

A person who is most active or creative during the night Wikipedia Disambiguation

A person who naturally stays up late at night Wikipedia Disambiguation

A person who tends to stay up until late at night Wikidata

A person who likes to stay up until late at night. OmegaWiki

One who goes to bed late, or stays up late at night or in the early hours of the morning. Wiktionary

A person whose preference or custom is to remain awake and active during the night and the early morning hours. Wiktionary