Sleep problems are quite prevalent in children and young people, and they are associated with the development of a range of poor outcomes in the short, medium and long term. Further, in most cases sleep problems in children do not occur in isolation but are the consequence of early life risk factors surrounding the child. In this presentation, I will share findings from my research in which we have explored how sleep patterns develop in children and adolescents, the factors contributing to sleep difficulties during childhood, and the impact of childhood sleep problems on health-related outcomes (primarily mental health problems) throughout childhood, adolescence, and early adulthood. For this, I will present results from my studies using secondary data analyses from existing birth cohort studies in Finland and in the UK. Finally, I will also present relevant methodologies that can be used with large longitudinal cohort studies to explore sleep development across several time points, as well as to investigate more complex associations between sleep and mental health, including potential underlying mechanisms.