Objective: Little systematic information is available regarding how sleep problems influence daytime functioning in children with ADHD, as the role of ADHD presentations and comorbidity is unclear. Method: In total, 397 children were assessed with the Children’s Sleep Habits Questionnaire, the Weiss Functional Impairment Rating Scale, and the ADHD Rating Scale. Results: We found a moderate, positive correlation between sleep problems and impaired functioning in both children with ADHD and in typically developed children. ADHD presentations did not differ significantly with respect to sleep problem profile, but having a comorbid internalizing or autistic disorder lead to higher sleep problem score. Conclusion: Sleep problems and impaired daily functioning were more common in children with ADHD, but the overall association between sleep problems and impaired daily functioning was similar in clinical and nonclinical children. Internalizing or autistic comorbid disorders added significantly to the sleep problems.