The study's aim was to investigate how school start time affects sleepiness and functioning in Norwegian 10th grade students (N = 106). The intervention school started at 0930 hours on Mondays and 0830 hours the rest of the week. A control school started at 0830 hours all schooldays. The students were assessed on a reaction time test as well as with self-report measures of sleepiness, mood and sleep. The intervention school obtained one hour longer total sleep time on Sunday nights compared to the control school and performed better on the reaction time test on Mondays than on Fridays, relative to the control school. Later school start times may both lengthen nocturnal sleep and increase students' alertness in early morning classes. (Contains 4 tables and 2 figures.)