Summer and SERT: Effect of daily sunshine hours on SLC6A4 promoter methylation in seasonal affective disorder

Objectives: Knowledge on how sunlight impacts SERT activity via SLC6A4 promoter methylation in Seasonal Affective Disorder (SAD) remains limited. This study aimed to investigate the effect of daily sunshine duration on SLC6A4 promoter methylation in 28 patients with SAD and 40 healthy controls (HC). Methods: Daily sunlight data for Vienna, Austria (mean of 28 days before blood sampling), were obtained from ©GeoSphere Austria. A general linear model analysed SLC6A4 promoter methylation as the dependent variable, with sunlight hours as the independent variable, and group (SAD, HC), age, sex, and 5-HTTLPR/rs25531 as covariates. Exploratory analyses examined the effects of sunlight hours and methylation on Beck Depression Inventory (BDI) scores. Results: Sunlight had a significant effect on SLC6A4 promoter methylation (p = 0.03), with more sunlight hours resulting in lower methylation (r = −0.25). However, the interaction between sunlight and group was non-significant, suggesting a rather general effect across both groups. Sunlight also influenced BDI scores (p < 0.01), with fewer sunlight hours leading to higher scores (r = −0.25), which aligns with previous research. SLC6A4 promoter methylation had no significant effect on BDI scores. Conclusions: Our findings suggest that sunlight influences SLC6A4 methylation without SAD specificity.