An epidemiological study of sleep-wake timings in school children from 4 to 11 years old: insights on the sleep phase shift and implications for the school starting times' debate.

Objective: It has been assumed that during adolescence there is a strong shift toward eveningness chronotype, whereas children's sleep is relatively stable. Several studies have focused on the conflict between school start times and adolescents' circadian rhythms; however, fewer studies have been conducted in younger children. The aim of this study was to examine sleep durations, schedules, and sleep phase shift in preschool- and school-aged children.

Methods: Data for sleep patterns on school days and free days (ie, days when children's sleep-wake patterns were free from any influence of individual or family activities) was obtained by means of questionnaires (Children ChronoType Questionnaire) for 3155 Portuguese children 4-11 years of age.

Results: As children grew older and school grade level increases, we found later bedtimes and sleep onsets on both school days and free days; and later wake times only on free days. By contrast, wake times were progressively earlier, imposed by school start times. There was a progressive reduction in the amount of sleep on school nights as grade level increased. Greater social jetlag, later midpoint of sleep, and higher restriction-extension patterns were found across age groups.

Conclusion: The displacement of bed and wake times for later hours on free days starts at an early age. Changing early school start times could adjust social demands to the biological rhythm of children.