This trial will assess whether long-acting levodopa taken at night improves obstructive sleep apnea (OSA) in patients with Parkinson's disease (PD), as compared with placebo.

Participants with Parkinson's disease and sleep apnea will take long-acting levodopa and placebo at bedtime for 2 weeks each, in a randomized order, with a 2-week washout period in-between. Sleep studies will be done at the end of each period. The primary outcome will be the apnea-hypopnea index change from baseline to on-medication, comparing active medication to placebo. Other outcomes will include other polysomnographic parameters such as oxygenation measures and sleep architecture variables, as well as the Epworth Sleepiness Scale, the Montreal Cognitive Assessment, the Parkinson's disease sleep scale and the Unified Parkinson's Disease Rating Scale. Adverse events will be closely monitored.

Inclusion Criteria:

1. Diagnosis of idiopathic PD consistent with the UK Brain Bank;
2. Presence of OSA on screening PSG, as defined by an AHI ≥ 15/h (moderate to severe OSA);
3. Stable regimen of anti-PD medication for 4 weeks prior to entry into the study, and no planned change during the study

Exclusion Criteria:

1. Other major neurological disorder;
2. Already taking long-acting levodopa (at any time of day);
3. Taking short-acting levodopa at bedtime or during the night;
4. Any contraindication to long-acting levodopa (see below);
5. Severe levodopa induced dyskinesias;
6. Already on or requiring treatment for restless legs syndrome ;
7. Body mass index >35 kg/m2;
8. Intercurrent upper respiratory tract infection;
9. Other known cause of OSA (e.g. craniofacial malformation);
10. Active treatment of OSA (CPAP, dental appliance or other), unless willing to stop treatment 2 weeks prior to start and during the duration of the study.