Effects of school start time and its interaction with the solar clock on adolescents' chronotype and sleep: A systematic review and meta-analysis.

Adolescents' late chronotypes colliding with early school start times (SSTs) are associated with students' unhealthy sleep habits. Most studies comparing different SSTs associate later SSTs with longer sleep duration and lower social jetlag. However, the magnitude of the effect varies between studies and the effect of different SSTs on chronotype is not well established. Importantly, although human circadian rhythms are entrained by sunlight, when studying the effect of different SSTs on adolescents' sleep habits usually only the social clock, and not the solar clock, is considered. This meta-analysis investigates whether later SSTs affect adolescents' sleep habits and chronotype and it assesses factors that can modulate this effect, including the relative importance of social and solar clocks. Here, through a database search we identify 37 studies comparing the effect of different SSTs on adolescents' sleep habits and/or chronotype. Random effect meta-analyses showed that later SSTs are associated with later sleep timings and longer sleep duration on weekdays, lower levels of social jetlag, and later chronotypes. Several meta-regressions reveal that the distance between compared SSTs and the interplay between SSTs and the solar clock modulate the effect of different SSTs on sleep timings and duration on weekdays.