Later school start times for supporting the education, health, and well-being of high school students (Cochrane/Campbell systematic review)

17 records reporting 11 unique studies, 297,994 participants

Students aged 13-19 in schools with differing start times, worldwide

Cochrane/Campbell systematic review restricted to RCTs, controlled before-and-after studies, and interrupted time series with sufficient data points. Searched October 2014, updated February 2016. Findings were synthesised narratively because the studies were too heterogeneous for pooling in most outcomes. The review's central methodological verdict matters more than any effect it reports: the overall quality of the body of evidence was rated VERY LOW, with most studies at high or unclear risk of bias for allocation, attrition, absence of randomization, and baseline data collection.

Six studies examined total sleep and reported significant positive relationships between later start times and sleep amount — this is the most consistent finding in the review. Four studies examined academic outcomes and reported MIXED results. Results for absenteeism were mixed; three studies on alertness were mixed; one study reported an association with decreased depressive symptoms. The authors conclude that "as a result of the limited evidence base, we could not determine the effects of later school start times with any confidence." They also note that RCTs here are near-impossible: school systems will not cede scheduling control to researchers. Limited evidence of adverse logistical effects (reduced parent-child interaction in one qualitative strand; staffing and scheduling difficulties in another).

The review does not frame confounding genetically, but its risk-of-bias findings are the same point in another vocabulary: absence of randomization and uncontrolled allocation across most included studies means selection into early- versus late-starting schools is not ruled out.