A one-hour delayed school start improves sleep, sleepiness and inhibitory control in early adolescents in a randomized controlled trial

Early school start times (SST) force adolescents to wake earlier than biologically preferred, creating chronic sleep debt. While evidence supports delaying SSTs, few controlled studies have used objective measures to assess their impact. We conducted a controlled intervention in a French boarding school to evaluate the effects of a one-hour delay in SST on sleep (N = 50) and cognitive functioning (N = 73) in early adolescents (age 12.8 years [11.7–14.2], 66% girls). After a baseline period with 8 a.m. starts (T0), four classes were randomized: half remained at the early schedule (Control-SST), half switched to 9 a.m. (Delayed-SST). Sleep measured by actigraphy, cognitive performance and mental health were assessed at baseline (T0) and 6 months later (T1). Between T0 and T1, total sleep time decreased in the Control-SST group, whereas the Delayed-SST group showed a modest, non-significant increase. This resulted at T1 in a 26-min between-group difference (Cohen's d = 0.93, p = 0.007). Sleep onset time did not differ between groups. Sleepiness decreased in Delayed-SST but increased in Control-SST (d = -0.52, p = 0.042). Inhibitory control improved in Delayed-SST compared to Control-SST (d = -0.79, p = 0.001), with trends toward better sustained attention (d = -0.40, p = 0.051). Delaying SST by one hour appears to mitigate the progressive reduction in sleep duration commonly observed during adolescence, and benefits both cognitive functioning and sleepiness outcomes in early adolescence.