AbstractBackgroundRecent research has discussed the possible explanations for the high frequency of excessive daytime somnolence (EDS) in Parkinson's Disease (PD). The conclusions remain unclear whether it is the disease itself, dopaminergic medications or other causes which lead to EDS.ObjectiveThe main objective is to explore the relationship between EDS and PD; what predicts daytime sleepiness in PD? Are the patients medicated on dopamine agonists (DA) sleepier than those who are not taking these medications? MethodsClinical and demographic information concerning pasients with PD were collected. Standardized questionnaires were used to assess the degree of parkinsonism (part 3 motor examination of the Unified rating scale for parkinsonism version 3.0 february 1987, UPDRS), sleep quality (The Parkinson´s disease sleep scale, PDSS) and daytime sleepiness (The Epworth sleepiness scale, ESS). The use of any dopaminergic medication were registered. Regression analysis and Chi-Square Tests were used to evaluate any possible connection between EDS and DA. ResultsThis cross-sectional study included 185 patients. The mean ESS score was 6.8 (SD 4.5), and 22.2 % had an ESS score of 10 or more, which was defined as EDS. There were no statistical significant differences in ESS score, duration of PD, age, motor UPDRS or sleep quality related to use/ no use of DA. Among the patients with highest scores on the UPDRS, the patients medicated on higher doses of DA were significant sleepier than the others (70 % had an ESS >= 10 vs 40.0 % on low doses and 20.8 % among those without DA; p < 0.001). There were no similar findings among the patients with the lower scores on the UPDRS.ConclusionOur findings suggest a causal connection between EDS in PD and the combination of the severity of the disease and the use of DA. Taking this into consideration, the doctors should be cautious when they prescribe DA to patients with advanced PD and be conscious that sleepiness among these pasients could be a sideeffect of DA.