The impact of timing on bright light therapy: Alleviating anhedonia and circadian rhythm disturbances in depression patients: a randomized controlled trial

This study aimed to determine the effectiveness of different times of bright light therapy (BLT) in alleviating anhedonia and circadian rhythm disturbances in depression patients. The study also explored whether the positive effects of BLT on anhedonia in depression were associated with changes in melatonin and cortisol indices throughout the treatment period. In this single-blind, placebo-controlled study, 81 patients were randomly assigned to one of four groups: morning BLT group, morning placebo group, evening BLT group, or evening placebo group. Participants in the BLT groups received BLT at 10,000 lx, while those in the placebo groups were exposed at 100 lx for 30 min daily over two weeks. The primary outcome measure was the Revised Physical Anhedonia Scale (RPAS) score. Secondary outcome measures included salivary melatonin and cortisol levels, as well as the Hamilton Depression Rating Scale (HAMD) score. The morning BLT group showed a significant delay in the melatonin peak phase and an improvement in RPAS scores compared to the morning placebo group. A correlation between changes in RPAS scores and melatonin peak phase was observed (P = 0.048) after morning BLT, with multiple linear regression analysis further confirming this relationship (P = 0.026). These findings suggest that morning BLT significantly alleviates anhedonia in depression, an effect associated with delayed melatonin peak phase. Depressive symptoms improved following both morning and evening BLT. These results highlight the potential of bright light therapy as a non-pharmacological treatment option, with morning administration showing particular benefit for anhedonia.