Several studies have reported a relationship between Attention Deficit Hyperactivity Disorder (ADHD) and sleep problems. Parents of children with ADHD report on decreased nocturnal sleep efficiency and sleep fragmentation compared to controls. On the other hand, objective measurements often show that these children sleep longer, and there is even a theory that suggests that daytime somnolence is the primary cause of the inattention problems seen in the disorder. Experimental sleep restriction, which leads to daytime somnolence, has been shown to be associated with ADHD-like behavior and poor cognitive achievements. In a study that we have recently performed, we found that children with ADHD had high prevalence of primary sleep disorders and objective daytime somnolence. This finding can explain why Ritalin, which is a stimulant medication, improves symptoms in these children. Sleep problems, primarily treatable such as Obstructive Sleep Apnea (OSA) or Periodic Limb Movements Disorder in Sleep (PLMD), should be investigated in the assessment of these children.