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?
We compare 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.
Our findings are the first to quantify the potential academic benefits of changing high-school start times—a seemingly straightforward policy that districts can find difficult to implement (see “How To Make School Start Later” in this issue). If districts in the Florida Panhandle started high schools later and elementary schools earlier, math and reading scores would increase by 6 percent and 4 percent of a standard deviation for high-school students, respectively, with negligible effects in younger grades. Rather than focusing on the disruptions associated with schedule changes, district and community leaders may wish to consider the ongoing costs of not adjusting schedules that are out of sync with sleepy teenagers’ physiological needs.
The two time zones in the Florida Panhandle provide an opportunity for a study on the effects of school start times. (Figure 1)
Sunlight, sleep, and puberty
The role of sunlight in determining sleep schedules is well known. In the morning, light on the outside of the eyelids suppresses production of the hormone melatonin and stimulates brain processes to increase alertness; darkness at night increases melatonin levels and feelings of tiredness. This process changes during adolescence: as children move through puberty, nocturnal melatonin secretion is delayed several hours relative to adults and younger children and sleep patterns become more owl-like, with later bedtimes and wake times, even holding the level of darkness fixed. Teenagers’ sleep patterns, therefore, are linked partially to sunrise and sunset times rather than to clock time.
This means that in terms of student sleep and alertness, the policy-relevant variable is “relative start times,” or start times relative to sunrise time. This becomes an important distinction when comparing schools in different locations, especially those near a time-zone boundary. Suppose that there are two schools close together but on opposite sides of the boundary, where the sun rises at 6 a.m. in central time and 7 a.m. in eastern time. If both schools begin classes at 8 a.m. local time, students attending the school in central time will have one more hour of sunlight before the morning bell compared to their neighbors in eastern time.
This scenario occurs in the Florida Panhandle, an area in the northwestern part of the state that includes the boundary between the eastern and central time zones (see Figure 1). While most of Florida is in the eastern time zone, the western half of the Panhandle is one hour behind in the central time zone. Thus, we have a natural experiment: do students living in the central time zone, who have as much as one additional hour of daylight before school each morning, do better in school than those living in the eastern time zone? Is this effect different for students of different ages? In particular, does it increase as students enter puberty?
To answer these questions, we track the academic achievement of individual students who move between schools on different sides of the time-zone boundary. As students move from central to eastern time, they are exposed to less sunlight before school, which we expect will decrease academic achievement. Conversely, students who move from eastern to central time gain sunlight before school and should see their test scores increase.
Data and methods
Our student and school data come from the Florida Department of Education administrative records for the 15 school years from 1998–99 through 2012‒13, and include all schools except alternative schools, adult education centers, and virtual academies. The data allow us to follow individual students over time, as long as they remain within the Florida public-school system. We look at scores on the annual Florida Comprehensive Assessment Test in math and reading, which students take in various years between grades 3 and 10, as well as students’ race, ethnicity, gender, eligibility for free or reduced-price school meals, and absentee rates. We use birthdays to calculate students’ ages at the start of the school year in September and include all students ages 8 to 15. We approximate students’ entry into puberty using the median age of 11 for girls and 13 for boys, according to data from the National Health and Nutrition Examination Survey.
Our study focuses on students who live near the time-zone boundary and make a substantial move, which we define as consecutive appearances at schools farther than 25 miles apart. Overall, the data show that movers are similar, but not identical, to students who do not move. They come from nearly identical schools, and those moving from west to east are similar to those moving from east to west. Compared to students who do not move, movers are 11 percent more likely to qualify for free or reduced-price school meals and have somewhat lower test scores—0.09 and 0.08 standard deviations lower in math and reading, respectively.
In looking at each group’s pattern of achievement in the years leading up to the move, we find that trends in test scores are similar for movers and non-movers. Among movers, the time until they move is also not a very strong predictor of academic achievement; that is, we find almost no difference in achievement between the year when a student moves and the year immediately before. This suggests that the groups are on similar underlying trajectories, and that any variation in post-move outcomes can be attributed to changes in sunlight before school.
Even so, it is the case that the vast majority of cross-boundary moves are over a great distance, and these moves may impact student outcomes independent of the change in relative start time. We address this concern by including in our analysis students who move schools, but not across the time-zone boundary. This allows us to disentangle the effect of moving from the effect of moving across a time-zone boundary.
To determine schools’ start times, we collected schedule data from their websites and followed up with phone calls where that information was not available. We exclude homeroom and breakfast programs to define school start time as the start of the first class where learning takes place, and find they range from 7 a.m. to 9:30 a.m. local time. The overall average in the Panhandle region is 8 a.m., in line with the average start time nationwide. Median start times for students vary by age: elementary students start at 7:55 a.m., middle-school students at 8:25 a.m., and high-school students at 7:50 a.m.
We use school-location data from the National Center for Education Statistics to calculate sunrise times for each school. Combining these with our start-time data, we average the difference over the school year before the testing date to construct a measure of relative start time, measured as the number of minutes between sunrise and school start times. Although school start times may differ across the time-zone boundary, they don’t differ enough to erase the one-hour difference in sunrise times.