Bright light therapy for depression and excessive daytime sleepiness in Parkinson’s disease: a systematic review and meta-analysis of randomized controlled trials with dose-response meta-regression

BACKGROUND: Non-motor symptoms, including depression and excessive daytime sleepiness (EDS), reduce quality of life in Parkinson's disease (PD). Pharmacological treatments may cause side-effects, highlighting the need for non-pharmacological approaches. We conducted an updated meta-analysis with linear and non-linear dose-response meta-regression to evaluate bright light therapy (BLT) on depression and EDS in PD. METHODS: PubMed, Scopus, and WOS were systematically searched through September 2025. Risk of bias was assessed using RoB-2 and NIH single-arm tools. Mean difference (MD) or standardized mean difference (SMD) with 95%confidence intervals (CI) were pooled using fixed-effects models. Linear and non-linear dose-response meta-regressions were performed. RESULTS: = 0.85) associations. Sensitivity analyses were consistent. CONCLUSION: BLT was not associated with significant improvement in depression or EDS in PD, and no dose-response relationship was identified. Given methodological limitations, BLT cannot currently be recommended for clinical use. Future standardized dose-finding trials stratified by disease stage and retinal/ipRGC integrity are needed.