Delaying high school start times impacts depressed mood among students: evidence from a natural experiment

2,134 high school students (mean age 15.16, SD 0.35) from the Minneapolis metropolitan area (START cohort)

US public high school students; two schools delayed start times, three comparison schools did not

Instrumental-variables analysis using the district-imposed start-time policy change as the instrument for a sustained gain in weekday sleep, with repeatedly measured Kandel-Davies depression symptoms as the outcome. The IV structure is the right one — it estimates the effect of sleep gained *because of* the policy rather than sleep chosen by the student — and the instrument is assigned at school level. Same cohort as Widome 2020 and James 2023, so these results are not independent replications of each other.

A sustained half-hour gain in weekday sleep attributable to the policy change decreased depression symptoms by 0.78 points, 95% CI (-1.32, -0.28), or 15.6% of a standard deviation, over two years. The benefit was driven almost entirely by the fatigue and sleep-related items of the scale. Symptoms of low mood, hopelessness, and worry were NOT affected on average — an honest decomposition that stops the result from being oversold as an antidepressant effect. Heterogeneity: older students with greater screen use and higher BMI improved more on mood symptoms, but universal implementation still outperformed targeted strategies.

Low for the instrument, which is assigned at school level by the district and is therefore orthogonal to student genotype. Medium for interpretation, because the outcome is self-reported and students knew their school had changed its schedule; the fatigue-item concentration of the effect is consistent with both a real mechanism and a reporting artefact.