Purpose of the Study: Sleep problems are frequently encountered in Parkinson's disease (PD), including sleep fragmentation, rapid eye movement (REM) sleep behavior disorder (RBD), excessive daytime sleepiness, and sleep-disordered breathing. In this study, we aimed to examine the relationship between sleep structure and sleep disorders on motor and nonmotor symptoms of PD. Basic Procedures: Seventy-three consecutive patients diagnosed as having PD based on the United Kingdom Brain Bank Criteria were prospectively enrolled. Detailed histories of PD-related symptoms, sleep anamnesis, subjective evaluation of nocturnal sleep, and daytime sleepiness were made. All participants underwent one-night video-polysomnography (PSG) and multiple sleep latency test (MSLT) in a sleep laboratory. Main Findings: A significant correlation was present between female sex and RLS (P = 0.009). Age and body mass index showed no significant correlations with PD-related parameters including Unified Parkinson's Disease Rating Scale (UPDRS) scores and PSG parameters. RLS or RBD showed no significant correlation with PD-related variables. Among PSG parameters, higher REM sleep percentages showed a statistically significant correlation with increased scores of UPDRS part III (P = 0.007). A statistically significant negative correlation was present between apnea-hypopnea index and PD duration (P = 0.005), and the presence of obstructive sleep apnea syndrome (OSAS) was statistically significantly correlated with lower scores of UPDRS part II (P = 0.050). The mean sleep latency in MSLT decreased as the dose of dopaminergic treatment increased (P = 0.016). Principal Conclusions: Our study demonstrated that changes in sleep structure and sleep-related disorders observed in PD could be attributed to intrinsic disease-related properties. The presence of changes in sleep structure as higher REM sleep percentages and sleep-related disorders such as OSAS show correlations with the severity of PD.