Bimodal effects of sunlight on major depressive disorder

Seasonal patterns in the effect of sunlight on depression, where depression decreases when sunlight increases, have been observed in previous studies. In this study, we demonstrate a bimodal effect of sunlight on depression - short-term increases in sunlight increase depression and long-term increases in sunlight decrease depression. The analysis showed that the significant effect of sunlight is temporary and appears only when seasonal changes are severe within a given year.

We analyzed approximately 530,000 cases where patients visited hospital for depression in Korea from January 1 to December 31, 2016. We measured the daily average amount of sunlight and daily sunlight for the 30 days previous to the day of measurement using data from 96 weather stations. To analyze the effect of sunlight, several climatic variables and local dummies were added to the negative binomial model, and the period in which the effect of sunlight was significant was derived as a term of the interaction between the month variable and sunlight.

When the average effects of climatic factors such as temperature, precipitation, and humidity were removed, the number of cases of depression increased when the daily average amount of sunlight increased [IRR = 1.024 (95% CI: 1.009 to 1.039)]; this effect was significant only in January and May. The number of cases of depression decreased with higher daily average sunlight for the previous 30 days increasing [IRR = 0.917 (95% CI: 0.892 to 0.944)], and this effect was significant only in January, March, and May.

The effect of sunlight on depression appears in both the short and long terms, but the effect is significant only for limited periods. The data examined in this study supports a pattern where short-term daily sunlight increases depression and daily sunlight for the previous 30 days decreases depression.