BACKGROUND: Excessive daytime sleepiness (EDS) is common in Parkinson's disease (PD), but its role and relation to other PD features is less well understood.

OBJECTIVE: To investigate potential predictors of EDS in PD and to explore how EDS relates to other motor and non-motor PD features.

METHODS: 118 consecutive persons with PD (54% men; mean age, 64) were assessed regarding EDS using the Epworth Sleepiness Scale (ESS) and a range of motor and non-motor symptoms. Variables significantly associated with ESS scores in bivariate analyses were used in multiple regression analyses with ESS scores as the dependent variable. Principal component analysis (PCA) was conducted to explore the interrelationships between ESS scores and other motor and non-motor PD aspects.

RESULTS: Among 114 persons with complete ESS data, significant independent associations were found between ESS scores and axial/postural/gait impairment, depressive symptoms, and pain (R(2) , 0.199). ESS scores did not load significantly together with any other PD features in the PCA.

CONCLUSIONS: Only a limited proportion of the variation in EDS could be accounted for by other symptoms, and EDS did not cluster together with any other PD features in PCAs. This suggests that EDS is a separate manifestation differing from, for example, poor sleep quality and fatigue.