Expression: Night owl

An owl is the kind of bird you see here. Owls are usually active at night, when they come out to hunt for food. They have specially-designed eyes which allow them to see well when many animals are not able to do so.

A night owl is a person who likes to stay awake late at night (sometimes all night) to work or play; in essence, someone who's more active at night than during the day.

We often talk about ourselves as morning people or night people. Personally, I'm more of a night person. How about you?

The opposite expression, for people who are very active in the morning, is early bird.