By Amy Norton
FRIDAY, Sept. 12, 2014 (HealthDay News) -- Sunny days may be linked to suicide rates, but in a complicated way, new research suggests.
In a study of more than 69,000 suicides spanning 40 years, Austrian researchers found two distinct correlations between sun-filled days and suicide rates. In the short term, sunny days were linked with an increase in suicide, but after two weeks of sunshine, the number of suicides dropped.
The findings, reported online Sept. 10 in JAMA Psychiatry, point only to an association between sunny days and suicide risk. And it's "impossible" to directly attribute the changes in suicide rates to sunshine, according to the researchers.
But in theory, there could be a role for the mood-regulating chemical serotonin, reported Dr. Benjamin Vyssoki and colleagues at the Medical University of Vienna.
A number of studies have found a seasonal pattern to suicide rates in several countries, including the United States -- but it's a counterintuitive pattern. Even though the dark days of winter can fuel depression in some people, suicide rates peak in spring.
So Vyssoki's team set out to see if sunlight, in particular, might be related to suicide rates. They looked at records for 69,462 confirmed suicides in Austria between 1970 and 2010, and used data from meteorological stations to calculate the average number of "sunshine hours" per day during the 40-year period.
Two patterns emerged, the authors said: In the shorter term -- 10 days or less -- more sunshine was tied to a higher suicide rate. But after a little more time -- 14 days to two months -- sunny days correlated with a lower suicide rate.
In addition, Vyssoki's team found that when they analyzed the data further, the short-term increase in suicide risk was mainly among women, while the longer-term decrease was largely among men.
What does it all mean?
It's hard to know, and the results should be interpreted cautiously, said Dr. John Mann, a professor of translational neuroscience at Columbia University Medical Center in New York City.
The factors that drive people to take their own lives are so complicated that pinning an effect on sunlight is fraught with difficulty, according to Mann, who was not involved in the study.
Another researcher who reviewed the findings said they add to "growing evidence of the importance of light in brain health."
"Sunlight and bright-light exposure have been associated with increased levels of serotonin in humans and animals," said Dr. Phyllis Zee, of the Center for Sleep and Circadian Biology at Northwestern University in Chicago.
That's one way that light might improve depression symptoms, Zee said. And in fact, light therapy is a treatment for people with seasonal affective disorder -- depression that strikes in the dark days of winter.
But why would suicide rates initially go up when the skies get brighter, then start to drop a couple of weeks later? The Austrian researchers speculated that serotonin transmission in the brain has a role.
Serotonin is implicated not only in depression, but also in impulsive behavior. And some scientists theorize that sunlight might affect some depressed people in a way that's similar to antidepressants: It may boost their "motivation" first, before improving their depression symptoms. That, in theory, could put some people with severe depression at increased risk of suicide for a window of time.
Mann stressed, however, that researchers are still trying to understand the effects that serotonin might have on the brain.
"It's hard to say that there's a plausible biological mechanism that would explain the correlations," Mann said.
And the researchers themselves noted that while they found correlations between sunny days and suicide, the "effect sizes were low."
"The effect would be very small, if it's there at all," Mann said. "I don't think people should avoid the sun because of this study."