Sunshine Duration, Genetic Predisposition, and Incident Depression: Findings from a Prospective Cohort

Background: Published studies have documented the association between sunshine duration and depression symptoms; however, the evidence regarding the long-term effects and potential mechanisms remains insufficient. This study aimed to examine the association between sunshine duration and incident depression and to explore potential mediating pathways. Methods: A total of 336,805 participants from the UK Biobank were included in the study. Meteorological exposures were estimated using the bilinear interpolation approach and time-weighted method. The association between sunshine duration and incident depression was examined through the time-dependent Cox proportional hazard model and generalized propensity score model. Vitamin D, calcium, immune biomarkers, an aggregated inflammation score (INFLA-score), and sleep pattern were selected as the potential mediators. Causal mediation analysis was employed to elucidate underlying mediating effects. Results: With a median follow-up of 13 years, 13,862 cases of incident depression were identified. Sunshine duration demonstrated a negative association with the incident depression. The effects were stronger among the elderly, alcohol consumers, individuals who spent less time outdoors, and those who were less physically active. Vitamin D, calcium, INFLA, neutrophils, and monocytes emerged as the top five contributors of immune biomarkers to the natural indirect effect. The combined mediating effect of top five biomarkers and sleep pattern accounted for 30% of the total effect of sunshine duration on the incident depression. Conclusion: Our study suggests that longer sunshine duration might mitigate depression through vitamin D-related metabolism, inflammation, and sleep pattern. It may serve as an effective natural antidepressant, particularly for the elderly, alcohol consumers, less outdoor spenders, and those who were less physically active.