Sleep problems are common in children with autism spectrum disorder (ASD), with poor sleep associated with core clinical symptoms, cognition, and daytime behaviours. Currently, there remain inconsistencies in findings across methods to assess sleep and few studies have examined the cognitive and behavioural sequelae of poor sleep in detail. The current thesis examined the tools used to assess sleep and investigate the relationships between sleep and daytime features of ASD. Overall, parental reports of sleep problems are high and enduring over time; however, few differences emerge between ASD and TD groups in objectively assessed sleep parameters. Given discrepancies, alternative explanations for the association between parent-rated sleep and daytime behaviours were considered.