Dopaminergic therapy is increasingly recognized as a cause of excessive daytime sleepiness in patients with PD. This adverse effect may be a dose-related phenomenon that is somewhat more likely to occur with dopamine agonists than with levodopa, although all dopaminergic drugs can be sedating. However, medication effect is only one of several causes of somnolence in PD. Other factors include age-related changes in sleep quality, nocturnal motor disturbances, primary sleep disorders such as sleep apnea, medication-induced sleep disruption, and concurrent medical illnesses. There is also increasing evidence that the disease process itself may affect the control of the sleep-wake cycle. Although we have characterized the sleep disturbances in PD, further investigation is needed to define their prevalence and etiology, particularly with respect to the role of dopamine and dopaminergic agents. Clinicians should be alert to the complaint of excessive sleepiness in their patients and should attempt to identify and treat the underlying causes.