Many U.S. high schools start classes before 8:00 A.M., yet research on circadian rhythms suggests that teenagers’ biological clocks shift to later in the day. This paper conducts the first study using a nationally representative dataset to examine the effect of high school start times on longer-run academic outcomes, including college-entrance exam scores and college attendance. Results indicate that female students who attend schools with later start times get more sleep and score higher on the SAT. Male students also get more sleep when their schools start later, but they are less likely to attend a four-year college.