Are you a lark—someone who greets the day before 7 a.m.—or a night owl who thrives after dark? Some 30% to 40% of the answer depends on your genes, says Philip Gehrman, director of the Sleep, Neurobiology, and Psychopathology lab at the University of Pennsylvania. But your internal clock is mostly determined by other factors, like your age, sex, environment—and distance from the snooze button.
Few people are extreme larks or night owls,
