Abstract
<title>Abstract</title> <p> Background Depression is one of the leading causes of disability worldwide, and its associated cognitive impairments severely compromise patients' occupational and social functioning. This study aims to evaluate whether bright light therapy (BLT) can improve mood and cognitive function in individuals with depression. Methods In this double-blind, placebo-controlled, parallel-group study, a total of 100 patients were recruited, of whom 90 inpatients were ultimately randomized and assigned to either a 10,000-lux BLT group or a 100-lux dim light control group. Emotional symptoms were assessed using the Hamilton Depression Rating Scale and the Hamilton Anxiety Rating Scale. Cognitive function was evaluated through event-based prospective memory, time-based prospective memory, and digit blinking tasks. Results Intention-to-treat analysis revealed that BLT was significantly superior to placebo in reducing depressive symptoms (between-group difference = -8.81 points, <italic>p</italic> = 0.003, <italic>Cohen’s d</italic> = 0.85) and anxiety symptoms (difference = -4.18 points, <italic>p</italic> = 0.013, <italic>Cohen’s d</italic> = 0.53) in patients with depression. Additionally, the BLT group demonstrated significantly greater improvement in both event-based ( <italic>p</italic> < 0.001, <italic>d</italic> = 0.71) and time-based prospective memory tasks <italic>(p</italic> < 0.001, <italic>d</italic> = 0.82) compared to the placebo group. Conclusions The results showed that BLT significantly improved mood symptoms and prospective memory in depressed inpatients compared with placebo. Meanwhile, the improvement in depressive symptoms was associated with enhanced prospective memory. Trial registration ChiCTR2500097364. Registered on 18 February 2025 at the Chinese Clinical Trial Registry (https//www.chictr.org.cn/). </p>