The mismatch between teenagers’ late sleep phase and early school start time results in acute and chronic sleep reductions. This is not only harmful for students’ learning in the short-term but may impact on students’ career prospects and widen social inequalities. Delaying school start times has been shown to improve sleep but whether this translates to better achievement is unresolved. The current evidence is limited due to a plethora of outcome measures and the many factors influencing sleep and grade/score trajectories. Here, we studied whether 0.5-1.5 years of exposure to a flexible school start system, with the daily choice of an 8AM or 8:50AM-start (intervention), allowed secondary school students (n=63-157, 14-19 years) to improve their quarterly school grades in a 4-year longitudinal pre-post design. We investigated whether sleep, changes in sleep or frequency of later starts predicted grade improvements in the flexible system. Our mixed model regressions with 5,111-16,724 official grades as outcomes did not indicate meaningful grade improvements in the flexible system per se or with previously observed sleep variables (nor their changes) – the covariates academic quarter, discipline and grade level had a greater, more systematic effect in our sample. Importantly, this finding does not preclude improvements in learning and cognition in our sample. However, at the ‘dose’ received here, intermittent sleep benefits did not obviously translate into detectable grade changes, which is in line with several other studies and highlights that grades are suboptimal to evaluate timetabling interventions despite their importance for future success.Significance statement Early school start times worldwide clash with teenagers’ delayed sleep-wake rhythms. This mismatch results in sleep restrictions below healthy amounts, which compromises health and performance and further aggravates social disparities. Since adequate sleep is important for learning and concentration, there is the strong expectation that counteracting sleep deprivation with delayed school starts results in better academic achievement. We add important high-resolution, longitudinal data to this unresolved scientific debate. When controlling for confounders, our results do not support that improved sleep leads to grade changes within 1.5 years in a flexible start system. While grades are suboptimal to measure later school start effects on performance, they nevertheless open doors to higher education worldwide and thus determine future trajectories.Competing Interest StatementThe authors have declared no competing interest.    «

The mismatch between teenagers’ late sleep phase and early school start time results in acute and chronic sleep reductions. This is not only harmful for students’ learning in the short-term but may impact on students’ career prospects and widen social inequalities. Delaying school start times has been shown to improve sleep but whether this translates to better achievement is unresolved. The current evidence is limited due to a plethora of outcome measures and the many factors influencing sleep...    »