The objectives of this study were to characterize the sleep of children with attention deficit hyperactivity disorder (ADHD), using actigraphy and parental questionnaires, and examine the potentially moderating role of psychostimulant medication and psychiatric comorbidity. Children with ADHD significantly differed from controls on parental and actigraphic measures of sleep, with parental reports indicating more severe sleep disturbances, and actigraphic recordings of longer sleep onset latency, lower sleep efficiency, and lower total sleep time. Both medicated and unmedicated ADHD subgroups differed from the control group on sleep measures, but did not differ from each other. Only the subgroup with comorbid psychiatric symptoms differed from the control group on actigraphic measures. The presence of psychiatric comorbidity, but not psychostimulant medication use, was associated with more severe sleep disturbances. The main implication of these findings is that clinicians should systematically attend to sleep disturbances in children with ADHD, particularly when other psychiatric symptoms are also present.