Background and Purpose: Sleep is an important determinant of health in adolescence, but many adolescents around the world are sleep-deprived. Compared to children and adults, getting enough sleep is especially challenging for adolescents due to the natural alteration of circadian rhythms that govern sleep-wake cycles. The delayed circadian rhythm does not necessarily lead to sleep deprivation unless late morning sleep is disturbed. Unfortunately, early school start times require adolescents to wake up early in the morning when their body "clocks" point to sleep time. As the negative impacts of inadequate sleep of adolescents have been increasingly recognized over the past decade, professional organizations, including American Academy of Pediatrics, have issued policy statements advocating secondary schools begin at 8:30 or later. With this increased attention to later school start times, many studies have investigated relationships between school start times and various students' outcomes. With respect to mood and substance use outcomes, while some correlational studies show a relationship between later start times and better mood and lower substance use, recent reviews suggest that strong evidence of the causal impact of school start time policies is missing. To fill this gap in knowledge, the present study exploiting a natural experiment in Korea that delayed school start times to between 8:30 and 9:00 am and leveraged a nationally-representative longitudinal survey to examine the impact of the policy on adolescents' sleep and substance use. Data and Participation: This study used the first to sixth waves of the Korean Children and Youth Panel Survey 2010 (KCYPS), which is longitudinal, nationally representative data of 7th graders in 2010. Administered by the National Youth Policy Institute, KCYPS annually tracked 2351 7th graders from 2010 to 2016, a year after they graduated from high school. KCYPS collected information on demographics, sleep, socio-emotional well-being, health, and developmental environment including family background. Importantly, this survey includes data on students' outcomes before and after a later school start time policy. Our outcomes of interest were sleep and substance use. The analytic sample was limited to students in either treatment regions or comparison regions, and students who moved across regions across waves were excluded. Since the final survey (i.e., 7th wave) was conducted after high school graduation when students were no longer exposed to the policy, it is excluded from the analysis. The final analytic sample sizes varied across models because some outcomes were surveyed only at some waves. To be specific, the final sample sizes were 6,324 observations from 1,133 students for models with sleep outcomes and 5,279 observations from 1,120 students for substance use outcomes. Policy Intervention: In September 2014, Gyeonggi province in Korea implemented a 9:00 am school start policy, which requires all elementary and secondary schools in the region to begin at 9:00 am. Following Gyeonggi province, some other regions implemented similar policies: Jeollabuk-do delayed school start time by 30 minutes in 2014, and Gwangju and Incheon delayed school start times to 8:30-9:00 and 8:40-9:00 am in 2015, respectively. Given that more than half of schools started before 8:00 am prior to policy adoption, school start times have been delayed approximately 30 minutes to one hour as a result of the policy. Research Design and Analysis: We employed a difference-in-differences (DD) design to estimate the impacts of later school start times on students' sleep and substance use, leveraging variation in a later school start time policy adoption across regions and years. Specifically, using a two-way fixed effect (TWFE) approach, which is an extension of the canonical 2X2 DD design, we compared changes in outcomes of adolescents before and after a later school start time policy adoption in treatment regions with changes in comparison regions. Our main model includes individual and year fixed effects to adjust time-invariant factors and general trends in outcomes. In addition to the main model, we estimated a model with time-varying covariates to account for time-varying individual-level factors. We clustered the standard errors at the region level to account for the nested characteristics of data and the level of treatment (i.e., students within regions) and account for heteroscedasticity. Also, we used sampling weights to make the analysis nationally representative. We conducted several supplemental analyses to better understand dynamic effects of later school start times and to check the robustness of our findings. First, we conducted event-study analyses of sleep and substance outcomes to identify the dynamic effect of later school start times and to examine the pre-treatment parallel trend assumption--that in the absence of a later school start time policy, trends in treatment and comparison regions would have been similar. In this model specification, we included pre-policy years (i.e., leads) and distinct post-policy years (i.e., lags) instead of one treatment indicator. Next, we incorporated recent advances in the econometrics literature to address potential bias in the TWFE approach. Findings: Adoption of a later school start time policy was initially associated with 20 minutes increase in sleep duration driven by delayed wake time and consistent bedtime. In addition, the policy was associated with statistically significant reductions in monthly smoking and drinking frequencies. However, approximately a year after implementation, the observed increase in sleep duration shrank to 6 minutes and became statistically nonsignificant. Similarly, the observed reduction in smoking and drinking was attenuated a year after. Conclusion: We demonstrated that policies that delay school start times can have health benefits beyond increases in sleep duration--getting more sleep can reduce unhealthy behaviors for adolescents. Our study also provides the first evidence of the benefits of delayed school start time policies in East Asia, and has insights on the sleep-health link for policymakers across Global regions. However, our results also serve as a warning to policymakers that, without measures to ensure the persistence of increased sleep, the benefits of delayed start times may be relatively short-lived.