Background: In the general population, sleep problems have an impact on daytime performance. Despite sleep problems being common among children with Down syndrome, the impact of sleep problems on daytime behaviours in school-age children with Down syndrome is an understudied topic. Our study examined the relationship between parent-reported and actigraphy-measured sleep duration and sleep quality with parent and teacher reports of daytime behaviour problems among school-age children with Down syndrome. Method: Thirty school-age children with Down syndrome wore an actigraph watch for a week at home at night. Their parent completed ratings of the child's sleep during that same week. Their parent and teacher completed a battery of measures to assess daytime behaviour. Results: Parent reports of restless sleep behaviours on the Children's Sleep Habits Questionnaire, but not actigraph-measured sleep efficiency, was predictive of parent and teacher behavioural concerns on the Nisonger Child Behaviour Rating Form and the Vanderbilt ADHD Rating Scales. Actigraph-measured sleep period and parent-reported sleep duration on the Children's Sleep Habits Questionnaire was predictive of daytime parent-reported inattention. Actigraph-measured sleep period was predictive of parent-reported hyperactivity/impulsivity. Conclusion: The study findings suggest that sleep problems have complex relationships to both parent-reported and teacher-reported daytime behaviour concerns in children with Down syndrome. These findings have implications for understanding the factors impacting behavioural concerns and their treatment in school-age children with Down syndrome.