Inclusion Criteria:  Diagnosis of RBD as assessed by the International Classification of Sleep Disorders (ICDS) diagnostic criteria within 3 years 40 yrs of age  Exclusion Criteria:  Other significant neurological disorder Untreated significant sleep apnea and/or current sleep apnea symptoms as assessed by the clinical investigator Co-existent restless legs syndrome (RLS), as assessed by the ICDS diagnostic criteria for RLS Cognitive impairment as determined by the Montreal Cognitive Assessment (MoCA) score of ≤ 23 Presence of depression defined as the Beck Depression Inventory (BDI) score >20 Use of tricyclic antidepressants, MAO inhibitors or selective serotonin reuptake inhibitors since they may induce/worsen RBD, unless on a stable dose of medication for at least 8 weeks Use of medications known to affect melatonin secretion, such as lithium, α- and β-adrenergic antagonists Shift work, currently or within the prior 3 months Travel through ≥ 2 time zones within 60 days prior to the screening