Later high school start times associated with longer actigraphic sleep duration in adolescents

383 adolescents (mean age 15.5, SD 0.6) with 1,116 school-day observations, Fragile Families & Child Wellbeing Study age-15 wave

US adolescents from a national birth cohort sampled across 20 large US cities; start times ranged 06:00 to 11:05

Multilevel models on concordant daily diary and actigraphy data, controlling for socioeconomic status. The measurement is excellent — objective actigraphy across a wide, diverse range of real start times — but the identification is purely cross-sectional. Start time is a property of the school the family happens to send the child to; nothing exogenous assigns it. Only ~2.9 days of data per adolescent.

Adolescents starting school at 08:30 or later had 21-34 minutes longer 24-hour sleep duration and 32-64 minutes later sleep offset than those with earlier start times. Analysed continuously, every 1-hour delay in start time was associated with 21 minutes longer 24-hour sleep, 16 minutes later sleep onset, and 39 minutes later sleep offset. Note that sleep onset moved later by 16 min here, unlike in the district-change studies where bedtime held roughly constant — consistent with these being different families rather than the same students under a changed constraint.

High. Sleep duration is roughly 69% heritable in adolescence, and the families who live in districts with late bell times differ systematically from those who do not. Controlling for socioeconomic status does not address passive gene-environment correlation. The magnitude here (21 min per hour) is notably smaller than the district-change estimates (34-45 min), which is itself a hint that cross-sectional and policy-induced variation are not measuring the same thing.