Delaying School Start in Adolescence (DESSA)

Early school start times force adolescents to wake before they are biologically ready. After puberty, a hormonal shift delays the circadian clock, making early rising harder. Yet most school schedules are set without accounting for this. The result is chronic sleep debt during a developmentally critical period, with documented knock-on effects on mood, cognition, and mental health.

We tested whether a simple one-hour delay in school start time, from 8 a.m. to 9 a.m., with no change to the end of the school day, was enough to make a measurable difference. Four classes in a French boarding school were randomised: two kept the early schedule, two switched to the later one. Sleep was tracked objectively using actigraphy over six months, alongside standardised measures of anxiety, sleepiness, and cognitive functioning. Students in the delayed group gained 26 more minutes of sleep per night than controls by the end of the study, without shifting their bedtime later. Anxiety and sleepiness decreased in the delayed group while increasing in controls. Inhibitory control improved significantly. These are large effect sizes by the standards of educational interventions, achieved through a scheduling change that requires no additional resources.

Preprint available online.