The American Academy of Pediatrics recommends that U.S. secondary schools begin after 8:30 a.m. to better align with the circadian rhythms of adolescents. Yet due to economic and logistic considerations, the vast majority of high schools begin the school day considerably earlier. We leverage a quasi-natural experiment in which five comprehensive high schools in one of the nation’s largest school systems moved start times forty minutes earlier to better coordinate with earlier-start high schools. Here, disruption effects should exacerbate any harmful consequences. We report on the effect of earlier start times on a broad range of outcomes, including mandatory ACT test scores, absenteeism, on-time progress in high school, and college-going. While we fail to find evidence of harmful effects on test scores, we do see a rise in absenteeism and tardiness rates, as well as higher rates of dropping out of high school. These results suggest that the harmful effects of early start times may not be well captured by considering test scores alone.