Age-standardized suicide rates, both sexes, 2012
"It could be that sunlight affects serotonin in the body, they say. Serotonin then may influence impulsivity, mood and aggression, which can play a role in suicidal behavior.
Sunlight may act like antidepressant medications that affect impulsivity first and then mood later on. The early affect on impulsivity may explain the increased suicide risk over a short period of time, and the delayed affect on mood may explain the lower risk over a longer time span.”