A lark, early bird, morning person, or (in Scandinavian countries) an A-person, is a person who usually gets up early in the morning and goes to bed early in the evening. The term relates to the birds known as larks, which are known to sing before dawn. Human "larks" may sleep from around 10 p.m. to 6 a.m. (or earlier), and tend to feel most energetic just after they get up in the morning. They are thus well-suited for working the day shift.
The opposite of the lark is the owl, often awake at night. A person called a night owl is someone who usually stays up late and may feel most awake in the evening and at night. Researchers have traditionally used the terms morningness and eveningness to describe these two chronotypes.
Till Roenneberg, a chronobiologist in Munich, has mapped the circadian rhythms of more than 200,000 people. Biological processes, including sleep-wake patterns, that display an oscillation of about 24 hours are called circadian rhythms