Local school districts often stagger daily start times for their schools in order to reduce busing costs. This paper uses data on all middle school students in Wake County, NC from 1999 to 2006 to identify the causal effect of daily start times on academic performance. Using variation in start times within schools over time, the effect is a two percentile point gain in math test scores -- roughly fourteen percent of the black-white test score gap. I find similar results for reading scores and using variation in start times across schools. The effect is stronger for students in the lower end of the distribution of test scores. I find evidence supporting increased sleep as a mechanism through which start times affect test scores. Later start times compare favorably on cost grounds to other education interventions which result in similar test score gains. (Contains 8 figures and 10 tables.)