Fatigue or excessive daytime sleepiness are a significant problem for at least one third of PD patients. The intro to a 2009 About.com article titled About Fatigue in Parkinson's Disease reads:
For many people with Parkinson's disease, fatigue is just as disabling and unpleasant a symptom as the motor slowing or the trembling. Fatigue undermines all kinds of daily activities and motor rehabilitation programs. It feeds into our emotional reactions to PD symptoms and makes them all the harder to bear. It undermines our ability to cope with the challenges PD presents to us each day and makes it more difficult to connect with others. Fatigue therefore can sometimes increase our social isolation as it saps us of the energy we need to step out the door and to interact with others. If your doctor has not asked you about your level of fatigue, but you're symptoms or have questions about it, please bring it up.
To read the entire article on this subject, visit http://parkinsons.about.com/od/signsandsymptomsofpd/a/fatigue_in_PD.htm.