Changing school start times: impact on sleep in primary and secondary school students

~28,000 students annually (grades 3-12) plus parent-proxy reports for grades K-12, surveyed before and for 2 years after a district-wide bell-time change

US public school students, grades K-12, in a single large district (~55% White, ~21% free/reduced lunch); elementary moved 60 min EARLIER, middle 40-60 min later, high school 70 min later

Longitudinal repeated cross-sections around a single district's simultaneous re-shuffling of all bell times, with no untreated comparison district. Outcomes are self- and parent-reported sleep, not actigraphy, which inflates uncertainty. Its value is scale and the fact that it captures the *tradeoff*: most districts buy a later high school start by moving elementary earlier, and this study measures both sides of that trade.

Middle school students gained 29 min and high school students 45 min of weekday sleep; both effects were maintained at the 2-year follow-up, and the proportion reporting sufficient sleep rose while clinically significant daytime sleepiness fell. Elementary students moved 60 min earlier LOST 11 min of sleep, though the percentage reporting sufficient sleep, poor sleep quality, or daytime sleepiness did not change. Benefits were similar across racial and free/reduced-lunch groups. The asymmetry is the point: younger children shifted earlier lose far less than adolescents shifted later gain.

Low-to-medium. The bell change was district-imposed on all students, so genotype cannot differ across the pre/post contrast within a grade. The absence of a comparison district means secular trends and any co-occurring district policy are confounded, and self-report introduces expectancy effects in a district that had publicly campaigned for the change.