The effects of a delayed school start time by one hour were examined at a boarding school in Hong Kong. Two cohorts of high school students (N = 228; 61.8% female) were recruited respectively before and after a school start time changed from 7:30am to 8:30am. Both cross-cohort and within-cohort longitudinal comparisons yielded significant increase in total sleep time. Cross-cohort comparison yielded improvement in sleep quality, insomnia, life satisfaction, and psychological distress. Longitudinal data suggested that the longer the additional sleep time, the better was sleep quality, day-time functioning, and subjective wellbeing.It is recommended that adolescents between the ages 13 and 18 should regularly get eight to 10 h of sleep each night (Carskadon
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