This study evaluates the impact of California’s SB 328, the first statewide mandate delaying school start times for middle and high schools, on adolescent sleep, mental health, and academic outcomes. Using YRBS, ATUS, SEDA, and SAT data, we apply difference-in-differences and matched DID methods. SB 328 led to significant improvements in sleep duration and academic performance. We find suggestive mental health benefits, though estimates are imprecise, and substantial heterogeneity in effects, with stronger gains among boys and Hispanic students across both sleep and academic outcomes.