To assess the efficacy of bright light therapy (BLT) in reducing depressive symptoms in patients with Parkinson disease (PD) and major depressive disorder (MDD) compared to a control light.MethodsIn this double-blind controlled trial, we randomized patients with PD and MDD to treatment with BLT (±10,000 lux) or a control light (±200 lux). Participants were treated for 3 months, followed by a 6-month naturalistic follow-up. The primary outcome of the study was the Hamilton Depression Rating Scale (HDRS) score. Secondary outcomes were objective and subjective sleep measures and salivary melatonin and cortisol concentrations. Assessments were repeated halfway, at the end of treatment, and 1, 3, and 6 months after treatment. Data were analyzed with a linear mixed-model analysis.ResultsWe enrolled 83 participants. HDRS scores decreased in both groups without a significant between-group difference at the end of treatment. Subjective sleep quality improved in both groups, with a larger improvement in the BLT group (B [SE] = 0.32 [0.16], p = 0.04). Total salivary cortisol secretion decreased in the BLT group, while it increased in the control group (B [SE] = -8.11 [3.93], p = 0.04).ConclusionBLT was not more effective in reducing depressive symptoms than a control light. Mood and subjective sleep improved in both groups. BLT was more effective in improving subjective sleep quality than control light, possibly through a BLT-induced decrease in cortisol levels.ClinicalTrials.gov identifier:NCT01604876.Classification of evidenceThis study provides Class I evidence that BLT is not superior to a control light device in reducing depressive symptoms in patients with PD with MDD.