Adolescents in high school suffer from circadian misalignment, undersleeping on weekdays and oversleeping on weekends. Since high schools usually impose early schedules, adolescents suffer from permanent social jetlag (SJL) and thus are a suitable population to study the effects of SJL on both academic and cognitive performance. In this study, 796 adolescents aged 12-16 years reported information about their sleep habits, morningness-eveningness (M-E), cognitive abilities and grade point average (GPA). Time in bed on both weekdays and weekends was not related to cognitive abilities, and only time in bed on weekdays was related to academic achievement. SJL was negatively related to academic achievement, cognitive abilities (except for vocabulary and verbal fluency abilities) and general cognitive ability (g), whereas M-E was slightly positively related to academic achievement and marginally negatively related to inductive reasoning. Results separated by sex/gender indicated that SJL may be more detrimental to girls' performance, as it was negatively related to a greater number of cognitive abilities and GPA.