Part I -- 1. Introduction: Sleepy days and restless nights of Parkinson's Disease -- 2. Regulation of sleep and wake homeostasis -- 3. Neurochemistry of the sleep-wake cycle in Parkinson's disease -- 4. Impaired sleep and alertness in Parkinson's disease: what did we learn from animal models? -- 5. Objective measures of the sleep-wake cycle in Parkinson's disease -- 6. Subjective assessment of sleep and sleepiness in Parkinson's disease. Part II -- 7. Excessive daytime somnolence associated with Parkinson's disease -- 8. Insomnia in Parkinson's disease -- 9. Sleep-disordered breathing in Parkinson's disease -- 10. Restless legs syndrome and periodic limb movement disorder in Parkinson's disease -- 11. REM sleep behavior disorder -- 12. REM sleep behavior disorder: a pre-motor marker of Parkinson's disease -- 13. Dysregulation of circadian system in Parkinson's disease -- 14. Cognition and the sleep-wake cycle in Parkinson's disease -- 15. Fatigue and sleepiness in Parkinson's disease -- 16. Impact of surgical therapies on sleep and alertness in Parkinson's disease -- 17. Future directions.