Exposure to sunshine over a short period of time may increase the risk of suicide but may also lower the risk over several months, suggests a new study from Austria.
People shouldn't avoid the sun based on the study's findings, says its senior author. Instead, they may consider adding sunshine to the list of factors that may influence the risk of suicide.
"Suicide is complicated and has many risk factors," said Dr. Matthaeus Willeit from the University of Vienna.
"People always tend to think of it in either biological or social terms, but there is no single cause," he said. "It's a bunch of risk factors that you have. That is just one of many risk factors."
Researchers have studied seasonal variations among suicides for centuries, with rates peaking in the spring. The actual effect of sunshine on those rates is less known, though. Along with changes in sunlight, new seasons bring temperature changes and a number of other factors that may influence suicide risk, such as holidays.
For the new study, the researchers used information on 69,462 suicides that occurred in Austria between January 1970 and May 2010. That data was then matched to data collected from 86 weather stations that recorded the hours of sunshine per day.
The researchers found a correlation between the number of suicides per day and the amount of sunshine throughout the study.
After adjusting the numbers to account for seasonal variations in suicides, they found that suicide risk went up with the amount of sunshine over the previous 10 days. Suicide risk appeared to decrease with increasing sun exposure between 14 to 60 days earlier, however.
The researchers suggest that sunlight may increase the risk of suicide over a short period but actually protect against it over a longer period of time. They can't definitively say sunlight causes or prevents suicides, however.
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 behaviour.
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.
"Light has an influence on serotonin and serotonin has an influence on mood and suicidality," Willeit said. "That's probably one of the biological links."
Alternatively, he and his colleagues write in JAMA Psychiatry, the early increased risk of suicide after sun exposure may lead to those most at risk to take their own lives. Fewer of the most at-risk people would then be susceptible to sun exposure later on.
As for right now, Willeit said that the study can't instruct doctors what to tell their patients based on weather reports.
"In the long term it would be great to know whose risk really increases with light," he said.