Less sleep in preschool-age children significantly predicted worse parent-reported hyperactivity and inattention at kindergarten. In contrast, hyperactivity and inattention at preschool did not predict sleep duration at kindergarten. The sample consisted of approximately 6,860 children, and analyses controlled for gender, ethnicity and family income. According to the authors, sleep problems, particularly difficulty falling asleep and staying asleep, are frequently reported in children and adolescents with ADHD. Longitudinal studies may reveal the direction of causation.