Light therapy in seasonal affective disorder: a systematic review and meta-analysis of randomized controlled and crossover trials

Background The efficacy of light therapy for seasonal affective disorder (SAD) has been supported in several studies. However, the duration of the effectiveness of light therapy at different time points has not been robustly examined. Objective To investigate the effect of light therapy at different time points of treatment and at post-treatment follow-up among individuals with seasonal affective disorder. Design A meta-analysis of randomized controlled and crossover trials. Method Articles were searched in nine electronic databases from inception to April 2025, and 17 eligible randomized controlled trials and crossover trials were included. The risk of bias for each included study was assessed using the criteria in the Cochrane Handbook for Systematic Reviews of Interventions, Version 2.0. Results Light therapy significantly reduced symptoms of seasonal affective disorder during week 2 (Hedges’ g = -0.62, p < 0.001), while the effect at weeks 1, 3, and 4 were non-significant (g = -0.17, p = 0.066, g = -0.60, p = 0.134, g = -0.15, p = 0.313). These effects correspond to estimated reductions of approximately 0.8, 3.1, 3.05, and 0.8 points, respectively, on the SIGH-SAD scale. At 1-week post-treatment follow-up, the maintenance effects of light therapy were not statistically significant for either 1-week (g = -0.52, p = 0.078) or 2-week (g = -0.36, p = 0.090) treatment durations. Measurement period was the significant moderator at week 2. Conclusion Light therapy can reduce symptoms of seasonal affective disorder, with a minimum of two weeks recommended to achieve desirable outcomes.