This systematic review evaluates the efficacy of bright light therapy (BLT) on depression and excessive daytime sleepiness (EDS) in Parkinson's disease (PD) patients through a meta-analysis and dose-response regression models.

Non-motor symptoms, including depression and EDS, are highly prevalent in PD patients and negatively impact their quality of life. Pharmacological treatments often cause side effects, highlighting the need for non-pharmacological approaches. BLT has been proposed as a potential treatment, but its efficacy remains uncertain.

A literature search was conducted through PubMed, Scopus, and Web of Science to identify RCTs evaluating BLT for PD. Quality assessment was done using RoB-2 tool. Meta-analyses were performed using R (meta and metafor packages), reporting mean differences (MD) or standardized mean differences (SMD) with 95% confidence intervals (CIs). Logistic dose-response meta-regression (linear and non-linear) examined the relationship between light illuminance and outcomes. Sensitivity analyses were performed to test the robustness of our findings.

Seven RCTs were identified, ranging from high to low quality, of which six were eligible for meta-analysis. For EDS, five studies with total 159 PD patients showed no significant effect of BLT (MD = –1.14, 95% CI –2.43 to 0.15, P = 0.08). Logistic dose–response analysis showed no linear (P = 0.78) or nonlinear (P = 0.97) relationships. For depression, five studies with total 194 PD patients found no significant effect (SMD = –0.13, 95% CI –0.41 to 0.16, P = 0.38), with no linear (P = 0.82) or nonlinear (P = 0.85) dose–response relationships. There was no significant heterogeneity for either outcome (I² = 0%). Results remained consistent in sensitivity analyses.

Current evidence does not support a significant effect of BLT on depression or EDS in PD patients, nor a dose-response relationship with light illuminance. Clarifying its therapeutic role requires larger, well-designed trials.