Sleep is vital for maintaining good mental health. Further, sleep and mental health problems are both public health concerns in their own right, with each having a substantive impact on both individuals and society. Adolescence and young adulthood are key periods in which to investigate mental health, as more than half of mental disorders start here. Similarly, sleep problems are often observed in young people. Further, it is crucial to understand under what circumstances and why sleep problems might lead to mental health problems, as this would shed some light on who are the young people most vulnerable for mental health problems. The proposed project will provide ground-breaking research on the prospective associations between sleep and mental health problems in young people using large longitudinal cohort studies, with a special emphasis on specific contributing and mediating factors of these associations. Among these, we will focus on social determinants (SD) (e.g., ethnicity, socio-economic status, housing conditions, food poverty) and cognitive factors as key aspects to consider in these associations in young people. To do this, we will use three population-based longitudinal cohort studies, which are the Avon Longitudinal Study of Parents and Children (ALSPAC) and the Millennium Cohort Study (MCS), both from the UK, and the Adolescent Brain Cognitive Development (ABCD), from the US. We will obtain detailed information on sleep (e.g., sleep duration, sleep fragmentation, sleep efficiency) using subjective (e.g., questionnaires) and objective (e.g., actigraphy) measures, mental health (e.g., anxiety, depression, psychosis, self-harm), cognitive factors (e.g., working memory, inhibition), and SD (e.g., ethnicity, living condition, early life adversities) in large sample sizes (e.g., >10,000 potential participants in each cohort).