Reports of sleep disturbances and delayed sleep timing in attentiondeficit/ hyperactivity-disorder (ADHD) are common however the aetiology of such features is poorly understood. There is substantial evidence pointing to dysfunction of the circadian timing system in ADHD, and individual differences in human chronotype and diurnal preference have been linked with impulsivity and attention problems in adults. In the work presented here we examined associations between a later circadian phase of entrainment, impaired sleep quality, and circadian misalignment and how they relate to core symptoms of ADHD distributed among the general population. We report novel evidence which suggests that ‘social jetlag’ – an index of circadian misalignment arising from discordance between endogenous circadian timing and the timing of the social clock – is a consistent predictor of poorer ADHD-like symptom outcomes. Furthermore, objective assessment of the rest-activity rhythm and sleep intervals of subjects show that a failure to precisely entrain to the 24 h circadian period is associated with ADHD-like symptom severity which was in turn predicted by delayed circadian phase/sleep phase, sleep quality, and duration. Candidate gene approaches did not replicate previous findings linking symptoms of impulsivity, inattention, and later chronotype with elements of the core molecular clock. However, we did find differential susceptibility to the previously identified risk factors; poor sleep quality and social jetlag which were both modified by genotype. Preliminary data from an exploratory study examining the neurophysiological correlates of response inhibition and selective attention revealed interesting patters of ERP elicitation in individuals with high levels of social jetlag. The current findings highlight how examination of sleep and circadian rhythm disturbances associated with ADHD may inform our understanding of the disorder risk and might in the future be factored into interventions designed for better symptom management.