Excessive daytime sleepiness (DS) is a common symptom of Parkinson's disease (PD), which can occur at any stage of the disease and decrease the quality of life of patients. The relationship between excessive daytime sleepiness and Parkinson's disease is determined by neurodegenerative process in the brain structures. However, there are many other factors, which can contribute to this phenomenon.To assess the prevalence and causes of DS as well as its impact on the quality of life of patients.Seventy patients (mean age 65.0±8.4 years) with PD, stage I-III, including 48 patients with DS and 22 without DS, were examined. All patients underwent clinical/neurological and neuropsychological examinations using a set of scales and polysomnography, if necessary.Sleep disorders are the most frequent non-motor symptoms of PD. The prevalence of subjective DS is determined for the first time. The disassociation between subjective and objective DS is observed in 32,5% of patients. Clinical combinations of DS with other sleep disorders are described. Taking dopaminergic medication contributes significantly to the development of DS and affective disorders (anxiety, depression) play a role in subjective DS.