Researchers at the University of Washington tracked sleep in sophomores at two Seattle high schools using wrist monitors before and after the district delayed its start time by 55 minutes, a natural experiment that let them measure the real effect rather than rely on survey estimates.
After the delayed start took effect, students gained a median of 34 extra minutes of sleep per night, driven almost entirely by later wake times rather than later bedtimes. Median grades in the students' biology class rose 4.5 percent, and attendance and punctuality improved at one of the two schools.
Because students did not simply stay up later to compensate, the extra sleep reflected a real increase in total rest rather than a shift in timing alone. The study compared different groups of students in two schools before and after a single policy change rather than running a randomized trial, so other factors tied to that same school year cannot be fully ruled out, though the sleep and grade changes tracked the schedule shift closely.
Read the original study
Dunster et al., Science Advances, 2018 · doi.org/10.1126/sciadv.aau6200
A later start bought real sleep, not bedtimes.
Source
- Dunster, G. P., de la Iglesia, L., Ben-Hamo, M., Nave, C., Fleischer, J. G., Panda, S., & de la Iglesia, H. O. (2018). Sleepmore in Seattle: Later school start times are associated with more sleep and better performance in high school students. Science Advances, 4(12), eaau6200. https://doi.org/10.1126/sciadv.aau6200