This study aims to determine the most effective dose of light therapy to improve sleep in people with Parkinson's Disease. Four groups of participants will receive bright-white or dim-red light therapy at different times throughout the day.

This is a 16-week, randomized, phase II, parallel-group, dose-selection clinical trial of LT in participants with PD and co-existing sleep disruption using a comparative selection trial design.

Inclusion Criteria:

1. Diagnosis of idiopathic PD as defined by the Movement Disorder Society Clinical Diagnostic Criteria for Parkinson's disease;
2. PD Hoehn and Yahr stage 2-4;
3. A score of 2 (mild) or above on the Sleep Problems question of the MDS-UPDRS Part 1;
4. Stable dose of all PD medications for at least 30 days prior to randomization;
5. Willingness to wear an Actiwatch and complete daily sleep logs;
6. Age 45 or above

Exclusion Criteria:

1. Atypical or secondary forms of parkinsonism;
2. Co-existent significant sleep apnea at screening, as determined by the PI's clinical assessment; adequately treated sleep apnea, as assessed by sleep apnea machine download (CPAP download) will be permitted;
3. Co-existent symptomatic restless legs syndrome (RLS) (as assessed by the International Classification of Sleep Disorders (ICDS) diagnostic criteria for RLS) at screening;
4. Cognitive impairment as determined by a Mini Mental State Examination score <25 at screening;
5. Presence of moderate depression defined as a Beck Depression Inventory II (BDI-II) score ≥20 at screening;
6. Current untreated hallucinations or psychosis (drug-induced or spontaneous) with a score of 2 or above on the Hallucinations and Psychosis question of the MDS-UPDRS Part 2;
7. Use of hypno-sedative drugs for sleep or stimulants, unless the participant has been on a stable dose for at least 60 days prior to the screening;
8. Ongoing or recent (within 30 days prior to screening) Cognitive Behavioral Therapy for Insomnia;
9. Use of antidepressants, unless the participant has been on a stable dose for at least 60 days prior to the screening;
10. Work hours between 10 PM and 6 AM, within 60 days prior to randomization or anticipated during the 16 weeks after screening;
11. Travel between 3 or more time zones within 45 days prior to study screening or anticipated such travel during the 16 weeks after screening;
12. Unstable or serious medical illness;
13. History of significant eye trauma or disease, retinopathy, or cataract Grade 4 that would significantly affect transmission or processing of light through either eye;
14. Current use, use at any time during study participation, or use within the 30 days prior to screening of photosensitizing or other medications that in the opinion of the investigator would interfere with the safety of the participant during the trial, including: amiodarone, porfimer, psoralens, chlorpromazine, quinidine, demeclocycline, temoporfin, tetracycline, fleroxacin, oral isoretinoin, voriconazole, nalidixic acid, thioridazine, St. John's wort, ofloxacin, and piroxicam;
15. Pregnant women will be excluded from participation; if a participant is pre-menopausal, a urine pregnancy test will be conducted at randomization to determine eligibility.

This trial will follow the NINDS policy for data sharing.