Disruption of sleep and alertness is one of the most disabling non-motor symptoms of Parkinson's disease (PD). Mechanisms leading to impaired sleep and alertness in PD are not well understood, and treatment options remain limited. The proposed research will examine markers of the circadian system, sleepiness and sleep quality in PD patients and healthy controls. Further, the project will examine effects of bright light exposure on circadin function, sleep and alertness in PD.

Inclusion Criteria:

1. Diagnosis of idiopathic PD as defined by the United Kingdom PD Society Brain Bank Criteria;
2. PD Hoehn and Yahr stage 2-4;
3. Pittsburgh Sleep Quality Index (PSQI) score >5 in group 1, and ≤5 in group 2;
4. Epworth Sleepiness Scale (ESS) score ≥10 in group 1, and <10 in group 2;

Control participants will be matched for gender and age with PD participants.

Exclusion Criteria:

1. Atypical or secondary forms of Parkinsonism;
2. Cognitive impairment as determined by the Mini-Mental State Examination (MMSE) score of ≤ 26;
3. Presence of depression defined as the Beck Depression Inventory (BDI) score >14;
4. Use of hypno-sedative drugs for sleep or stimulants; participants will be allowed to taper these drugs and will become eligible at least 4 weeks after the taper is completed;
5. Use of SSRIs / SNRIs antidepressants, unless the participant has been on a stable dose for at least 3 months prior to the screening;
6. Use of medications known to affect melatonin secretion;
7. Unstable/serious medical illness.