Sunlight and suicide risk: mediating effect of high temperature in China

Studies worldwide have reported an increase in suicide during hot weather. In summer, high temperature and intense sunlight often coincide, with the latter being a prerequisite for the former formation. However, little is known about whether and to what extent high temperature mediates the effect of sunlight on suicide. We obtained daily suicide death data in Anhui Province of China for the period from January 1, 2017 to December 31, 2020. Both daily sunshine hours and daily solar radiation were employed to measure the sunlight exposure. The space-time-stratified case-crossover analysis was used to estimate the association between sunlight exposure and suicide deaths in high- and non-high-temperature weather during the warm season (May through September). A linear regression was then utilized to estimate the extent to which the association was mediated through temperature. Sunlight during the warm season demonstrated a protective effect on suicide in non-high-temperature weather but a hazardous effect in high-temperature weather. Further mediation analysis in high-temperature weather showed that the direct effect of sunlight on suicide deaths was more pronounced than the effect mediated through temperature. The proportion mediated through temperature in the association between sunshine hours and suicide deaths ranged from 11.27% to 18.37%, while the range for solar radiation and suicide deaths was lower (0.50% to 0.89%). Sunlight not only affects suicide risk but also has an indirect effect that is partially mediated through high temperature. Our findings need to be proven in distinct regions, aiming to better understand the role of sunlight and high temperature in triggering suicide.