How school start times affect academic performance

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American teenagers are chronically sleep-deprived. As children enter puberty, physiological changes delay the onset of sleep and make it more difficult to wake up early in the morning. By the end of middle school, there is a large disconnect between biological sleep patterns and early-morning school schedules: one study found that students lose as much as two hours of sleep per night during the school year compared to the summer months, when they can better control their sleep schedules.
Such deficits may have big implications for learning and cognition. Important memory formation and consolidation processes occur overnight, as the brain replays patterns of activity exhibited during learning. Insufficient sleep also reduces alertness and attention levels the next morning, which likely affects students’ ability to learn. Both the American Academy of Pediatrics and the American Academy of Sleep Medicine recommend that high school start no earlier than 8:30 a.m. But most U.S. high schools—87 percent, at the last count in 2015—begin earlier.

Could something as simple as changing when school starts each day really make a difference in how much students learn? And which students would benefit most from a later start time? We consider differences between sunrise and school start times among a group of public schools in northern Florida’s “Panhandle,” which straddles the central and eastern time zones. In this region, sunrise times differ, but school start times do not fully adjust for this difference. Students may start school at the same hour on the clock but not at the same “time”—those in the later time zone could have as much as one additional hour of early-morning daylight before school compared to their neighbors in the earlier zone. How does this affect their performance in school?

This study compares test scores for students between the ages of 8 and 15 who move from one time zone to the other and find substantial differences, especially for adolescents. A one-hour delay in start times relative to sunrise increases math scores by 8 percent of a standard deviation for adolescents—the equivalent of roughly three months of student learning—but by only 1 percent of a standard deviation for younger children. The effects on reading scores are similar, but smaller. The benefits of starting school later increase sharply at age 11 for girls and 13 for boys—the gender-specific ages when puberty typically begins, which we take as evidence that the causal pathway is linked to biological changes that affect students’ sleep.