Abstract: Our search located 17 eligible records reporting on 11 unique studies with 297,994 participants; the studies examined academic outcomes, amount and quality of sleep, mental health indicators, attendance, and student alertness. Overall, the quality of the body of evidence was very low, as we rated most studies as being at high or unclear risk of bias with respect to allocation, attrition, absence of randomization, and the collection of baseline data. Therefore, we cannot be confident about the effects of later school start times. Preliminary evidence from the included studies indicated a potential association between later school start times and academic and psychosocial outcomes, but quality and comparability of these data were low and often precluded quantitative synthesis. Four studies examined the association between later school start times and academic outcomes, reporting mixed results. Six studies examined effects on total amount of sleep and reported significant, positive relationships between later school start times and amount of sleep. One study provided information concerning mental health outcomes, reporting an association between decreased depressive symptoms and later school start times. There were mixed results for the association between later school start times and absenteeism. Three studies reported mixed results concerning the association between later school start times and student alertness. There was limited indication of potential adverse effects on logistics, as the qualitative portions of one study reported less interaction between parents and children, and another reported staffing and scheduling difficulties. Because of the insufficient evidence, we cannot draw firm conclusions concerning adverse effects at this time. It is important to note the limitations of this evidence, especially as randomized controlled trials and high-quality primary studies are difficult to conduct; school systems are often unwilling or unable to allow researchers the necessary control over scheduling and data collection. Moreover, this evidence does not speak to the process of implementing later school starts, as the included studies focused on reporting the effects rather than exploring the process. This systematic review on later school start times suggests several potential benefits for this intervention and points to the need for higher quality primary studies. However, as a result of the limited evidence base, we could not determine the effects of later school start times with any confidence.

Plain-language summary: The evidence included in this review is current as of February 2016 and is based on 17 reports representing 11 unique studies in 297,994 high school students. The studies examined a range of changes to school time (for example, moving the start time fifteen minutes later, moving the start time an hour later) and a range of intervention durations (one as short as two weeks, others lasting a year), but all focused on natural settings (students already in schools, rather than in a laboratory setting). Although 5 of the 11 studies were funded, the funding sources were academic and research institutions, rather than agencies with a commercial interest in program evaluation results. Because of the limited and very low-quality evidence, we could not determine the effects of later school start times with any confidence. We found that later school start times may provide academic benefits, but results of four studies provided mixed findings. Later school starts were associated with an increase in school-night sleep for students based on the synthesis of two studies, and evidence from six other studies also supported the relationship between later school starts and increased sleep duration. One study reported that students in later starting schools reported fewer depressive symptoms than their peers in earlier starting schools. Different studies reported mixed findings regarding the association between later school start times and increased attendance and student alertness. These interventions may also have potential adverse effects on logistics, as the qualitative portions of one study reported less interaction between parents and children, and another reported staffing and scheduling difficulties. Again, because of the limited and very low-quality evidence, we cannot draw any firm conclusions about the adverse effects of later school start times. The quality of this evidence was very low, and thus we cannot assume the findings reflect the true beneficial or adverse effects of later school start times.