A critical period when sleep is likely to be under greatest pressure is during the senior high school years when academic demands are particularly intense with an increasing emphasis on homework. Sleep habits vary with culture as do attitudes toward academic success. What is unclear is how well students from different cultural ackgrounds balance the competing demands of study and sleep and the potential consequences on their mental health. In this study, 101 Asian-Australian and 297 Caucasian-Australian senior year high school students completed a standardized omnibus questionnaire examining extracurricular activity (homework, paid work, sports, exercise, and other organized activities), sleep habits, and mental health (depressive mood, study/ interpersonal stress, and coping). Analyses revealed that sleep habits and extracurricular activity varied with culture. These differences were most evident on school nights where Asian-Australian compared to Caucasian-Australian adolescents reported later bed and rise times and shorter sleep lengths. Although the total amount of time spent on extracurricular activity was similar, Asian-Australian adolescents undertook significantly more homework after school while Caucasian-Australian adolescents spent more time undertaking paid work, sports, and exercise. Regression analyses revealed that extracurricular activity including homework was not predictive of sleep length in either Asian or Caucasian adolescents. Further analyses revealed that predictors of mental health were most evident in Caucasian-Australian adolescents. In the latter group, female gender, less time spent on non homework activities, shorter sleep and daytime sleepiness were associated with depressed mood while female gender, increased homework time and daytime sleepiness were associated with increased study/interpersonal stress. These findings highlight the importance of culture in understanding the role of extracurricular activity on sleep and mental health in senior school students.