As expected, students obtained significantly less sleep during Time 2 (school-term period) than they did at Time 1 (holidays from school), particularly on weekdays, obtaining an average about 1 hour and 17 minutes less sleep than during holiday periods. Also as expected, students tended to wake up considerably earlier during the school term in order to meet school time schedules. Students woke up approximately 2 hours and 30 minutes earlier during the school term and showed larger variability between weekend and weekday sleep patterns during the school time compared with holidays. The study found that trait circadian preference mediated mood, daytime functioning, and academic grades through its effect on sleep variables during school periods. The study concluded that although the imposition of school schedule negatively impacted on mood and daytime functioning for the sample as a whole, evening-oriented adolescents were the most vulnerable to poorer outcomes. These students obtained poorer quality and less sleep than morning-oriented students. Sleep factors impacted negatively on evening-oriented students’ mood and ability to function at school during the day, which in turn predicted poorer academic achievement. The sample members were attending three metropolitan secondary schools in grades 11 and 12. A self-report survey adapted from the School Sleep Habits Survey was designed for the study. Scales contained a sleep log and measured grades, sleep quality, times going to sleep and waking, daytime functioning, and moods and feelings. 1 figure, 5 tables, and 36 references