Rise and Shine: The Effect of School Start Times on Academic Performance from Childhood Through Puberty

Florida statewide administrative student-test panel; identification uses the Florida time-zone boundary

US public school students spanning childhood through adolescence, Florida

Instrumental-variables design. Because school start times do not adjust to compensate for differences in sunrise, hours of daylight before school vary sharply across the time-zone boundary that runs through the Florida panhandle. The authors instrument for pre-school sunlight with that boundary. This is a genuinely clean source of variation — nobody selects a residence on the basis of within-state time-zone position for sleep reasons — and it isolates the biological-timing channel rather than the bell-time-policy channel. Limitation: the instrument identifies the effect of clock-time-relative-to-sunrise, which is related to but not identical to the policy lever a district actually controls.

Moving start times one hour later relative to sunrise increases test scores by 0.08 SD in math and 0.06 SD in reading for adolescents. In math the effect is larger for older children and co-varies with entry into an important pubertal stage — direct evidence that the mechanism is developmental circadian phase delay rather than generic "more sleep is good". The policy implication the authors draw is a reallocation, not an addition: districts can improve performance at zero cost by moving classes EARLIER for younger children and later for older ones, holding the distribution of start times fixed.

Low. Position relative to a time-zone boundary is not chosen by families for reasons correlated with child ability, and the IV strips out school-level and district-level selection into start times. This is one of the two cleanest achievement-side identifications in the literature (with Edwards 2012), and it is positive but small.