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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 (in some cases, until around dawn), and sleep until relatively late in the day. People with delayed sleep phase syndrome are often described as night owls.
The opposite of a night owl is an early bird — a lark as opposed to an owl — which is someone who tends to begin sleeping at a time that is considered early and also wakes early. Researchers traditionally use the terms morningness and eveningness for the two chronotypes, or diurnality and nocturnality in animal behavior. In several countries, especially in Scandinavia, one who stays up late is called a B-person, in contrast to an early riser being called an A-person.