We investigated whether (a) short and irregular sleep are related to aggressive behavior in early adolescence and (b) whether they moderate the relation between interparental conflict and aggressive behavior. 176 early adolescents (mean age 11.6 years, 89 girls) reported their bed and wake times on weekdays and on weekends and their aggressive behavior. Interparental conflict was rated by their mothers and adolescents reported whether they felt threatened by the conflict. No direct association between early adolescents' sleep and aggression was found. However, short sleep duration on weekday nights, long sleep duration on weekends, and a large weekday to weekend difference in sleep duration moderated the relation between interparental conflict and early adolescents' aggression. Findings are consistent with the hypothesis that irregular sleep is a vulnerability factor for early adolescents' aggression and underscore the importance of sufficient and regular sleep for resilience. (Contains 2 tables and 1 figure.)