About half of people with Parkinson's disease complain of fatigue. Some feel sleepy, whereas others report low energy or an exhaustion that levels them. Doctors will explore to what extent the symptoms result from depression or sleep problems; addressing those disorders may relieve the fatigue. Exercise may also help, so Friedman encourages patients to slowly work up to a regular walking routine of 30 minutes a day. If fatigue still persists, he sometimes prescribes stimulants.

As Parkinson's disease becomes an ever-greater obstacle to everyday routines, roughly 40 percent of patients exhibit apathy. They lose interest and joy in the hobbies or activities that used to bring them pleasure, and they seem indifferent to the friends, family, and events around them. They stop caring about anything.This can be upsetting to see in a patient, especially because it seems as if he's giving up on treatment and on life. Then again, apathy may protect him from distress and frustration over his growing disability, Friedman points out. Apathy may be related to depression, dementia, or sleep disorders, so addressing those conditions may help. Generally, however, there's no good cure for apathy.

Most people with Parkinson's disease will at some point experience mild cognitive changes that may hinder attention, memory, mental processing speed, problem-solving, processing of visual information, and the ability to pull the right words from one's vocabulary. In many cases, patients compensate for these subtle deficits without much detriment to life at work or home.