Background: In Northern latitudes, winter is the darkest time of the year, and depressive episodes during winter are prevalent. Although changing weather patterns due to climate change are projected to result in warmer and wetter and, thus, even darker winters, research on the impact of winter-time natural light and precipitation on mental health is scarce. We examined associations of exposure to solar radiation and precipitation with psychotropic medication and antidepressant purchases in winter months.

Methods: Of the 251,268 eligible participants from the Finnish public sector study, aged ≥18 years, 72% were women. Associations for municipality-level 4-week average solar radiation and precipitation with register-based medication purchases from 1999 to 2016 were analyzed using random effects method with Poisson regression. A 6-month washout period with no purchases was applied to each purchase. Confounding by region and year, and effect modifications by sex, age, and socioeconomic status were examined.

Results: No association was observed for an increase in 4-week average of solar radiation by standard deviation (585 kJ/m²) with any psychotropic medications (incidence rate ratio: 0.99; 95% confidence interval: 0.98, 1.00) or antidepressants (1.00; 0.99, 1.01). No difference in any psychotropic medication or antidepressant purchases in participants exposed to high solar radiation (≥2000 kJ/m²) compared with those with the lowest exposure (<500 kJ/m²) was observed. No associations were observed for precipitation.

Conclusion: No evidence linking higher solar radiation exposure to reduced psychotropic medication purchases, nor higher precipitation exposure to increased medication purchases in winter was observed. Further research is needed to validate and expand upon these findings.