0299 Chronotype and School Start Time as Determinants of Adolescent Social Jetlag: A Cross-Sectional Analysis

Abstract Introduction Social jetlag (SJL)—the misalignment between endogenous circadian rhythms and externally imposed schedules—has emerged as a critical concern in adolescent sleep research. Although California’s delayed school start mandate aims to mitigate sleep deprivation, the relationship among start time, chronotype, and SJL remains underexplored in U.S. adolescents. Moreover, subclass differences across grade levels have not been thoroughly investigated. This study examines how school start time and chronotype affect SJL, sleep duration, and bedtime variability among high school students in Fresno, California (late-start) and Peoria, Illinois (early-start). Methods A cross-sectional online adaptation of the Munich Chronotype Questionnaire (MCTQ) was administered to 632 students (grades 9–12) from early-start (7:30 AM; n = 222) and late-start (8:30 AM; n = 410) schools between December 2024 and February 2025. SJL was defined as the absolute difference between mid-sleep on school-free and school nights. Chronotype classifications (morning, intermediate, evening) were based on mid-sleep on free days. T-tests compared SJL, sleep duration, and bedtime variability across start times, grade levels, and chronotypes (p < 0.05). One-way ANOVA assessed differences in SJL across chronotypes. Results School start time alone did not significantly reduce SJL (p = 0.776). Mean SJL across all participants was 4.76 hours—substantially higher than in prior non-U.S. adolescent cohorts (1.5–2.5 hours)—suggesting cultural or systemic amplification of circadian misalignment. Chronotype was a major determinant: evening types showed greater SJL than morning types (p < 0.05), and ANOVA confirmed significant differences among all three chronotypes (p < 0.05), with intermediate types displaying intermediate SJL. Grade level also influenced outcomes: upperclassmen had shorter weekday sleep duration (p = 0.006) due to later bedtimes, while weekend sleep duration did not differ significantly (p = 0.169), indicating limited recovery. Conclusion Addressing adolescent sleep deprivation may require more than policy reforms. Interventions tailored to chronotype(especially for evening-type upperclassmen)and strategies that address behavioral contributors to sleep loss could be more effective. Future research should consider regional and cultural influences when designing sleep health interventions in U.S. adolescents. Support (if any)