A number of high schools across the United States have moved to later bell times on the belief that their previous bell times were too early for the "biological clocks" of adolescents. This paper studies whether doing so improves academic performance. I first focus on the Twin Cities metropolitan area, where Minneapolis and several suburban districts have made large policy changes but St. Paul and other suburban districts have maintained early schedules. I use individual-level ACT test score

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... a on all individuals from public high schools in this region who took the ACT between 1993 and 2002 to estimate the effects of school starting times on ACT scores. I then employ school-level data on starting times and test scores on statewide standardized tests from Kansas and Virginia in order to estimate the effects of bell times on achievement for a broader sample of students. The results do not suggest an effect of school starting times on achievement. This finding is unchanged by numerous robustness checks, and the Minnesota results are especially precise.