Disordered sleep is commonly reported in autism from a young age, as are symptoms of mental illness. Mental health and sleep are known to influence each other in a bidirectional manner within the general population, but the nature of the association between sleep and mental health problems in autism is poorly understood. Moreover, the underlying causes of sleep problems in autism are unknown. Thus, current treatments for sleep problems are largely ineffective in autistic people, and they also face severe consequences of mental ill health, such as high rates of suicidality and in-patient care. We aim to characterise the relationship between sleep problems and mental ill health, and identify factors underpinning the sleep problems in autism and in people with autistic traits. We will focus specifically on childhood and adolescence as this comprises the typical age-range of onset for problems in both sleep and mental health. Our approach to answering these questions will involve statistical modelling of longitudinal data on sleep and mental health in ALSPAC.