A study from Journal of American Medical Association Psychiatry found sunshine has a complicated relationship with suicide. Specifically, they found a negative correlation between the number of suicides and daily hours of sunshine for the 14 to 60 days prior to the suicide event and a positive correlation on the day of and the 10 days leading up to the event. This relationship was independent of season.
Research on the influence of season on suicide rates dates back to the early 19th century. A seasonal pattern for suicides has been found in many countries, with most suicides occurring in the spring. In Belgium, for example, the incidence of suicides was positively associated with sunshine duration, temperature, and an increase in temperature during the weeks prior to suicide.
Scientists studying this phenomenon believe that neurobiological effects of sun exposure offer the best explanation for seasonal variations in suicide rates. Ultraviolet light has been shown to interact with the serotonin system in humans and may influence serotonin-related behaviors, including mood, impulsiveness, and aggression. These behaviors have also been shown to play a key role in suicidal behavior.
Studies examining the effects of sunlight on suicidal behavior and suicide rates have almost all been limited by confounding variables. For example, seasons affect several climatic variables and social behaviors, such as holidays, anniversaries, unemployment rates, and availability of clinicians, which may also influence suicidal behavior. Therefore, it is hard to isolate the effects of sunshine on suicide behavior.
Researchers from Austria decided to study the effect that sunlight has on suicide rates independent of other factors that change with the seasons.
They collected data on suicide rates from January 1st, 1970 to May 6th, 2010 from Statistics Austria. Statistics Austria is an institution that collects data on various epidemiological information in Austria.
Data on the average duration of sunshine per day (in hours) for the same 40 years were calculated from 86 meteorological stations in Austria.
After running analyses on all the data and adjusting for confounding variables due to seasonal variation, here is what the researchers discovered:
- The mean sunshine duration and the mean number of suicides on each day from January 1st, 1970, to May 6th, 2010, were strongly positively correlated. (p<0.001)
- There was a positive correlation between number of suicides and hours of daily sunshine for the day of the suicide and up to 10 days prior to suicide. (p<0.001)
- There was a negative correlation between number of suicides and hours of daily sunshine for the 14 to 60 days prior to the suicide. (p<0.001)
These results suggest that sunshine has a biphasic effect, as both a short-term increase and a long-term decrease in suicides was seen. This indicates that through some mechanism, sunshine during the week leading up to the suicide somehow contributes to the event and that sunshine during the months leading up to the suicide are protective against the event.
Another possible mechanism that partially explains the results seen here as well the results of similar previous research is vitamin D status and vitamin D-mediated neurological effects. Vitamin D receptors and metabolites have been found in the region of the brain related to reward and pleasure, which indicates that vitamin D has an effect on these regions. Vitamin D status is directly related to the amount of daily sunshine, which may explain the long-term protective effect of sunlight against suicide events.
The researchers stated,
“In line with our hypothesis, we were able to show that sunshine has an effect on suicides independent of seasonal rhythms. Two main results emerged from our data. First, sunshine on the day of suicide and up to 10 days prior to suicide seems to facilitate suicide, as we found a positive correlation between the duration of sunshine and suicide numbers in this period. Second, to our knowledge, this is the first study to show that sunshine, after removal of seasonal variance in suicide numbers and sunshine duration, may also have a protective effect against suicide.”
The effect sizes of the results were small. Also, the researchers could not include data on multiple factors relating to suicide, such as genetic variables, mental health, lifetime substance abuse, and social factors.
This study was only designed to show correlation and therefore an absolute statement of the existence of causality, let alone the direction of causality, can’t be made.
One factor not accounted for in this study was the vitamin D status of the individuals examined. As mentioned above, vitamin D status may be indicative of greater sun exposure over time. While there may be other protective factors of sunlight, it may be that raising vitamin D levels to sufficiency and maintaining them over the course of months could be a marker contributing to the protective effect.
We know with fairly strong certainty that vitamin D plays a role in depression. Furthermore, one previous study found a relationship between vitamin D deficiency and military suicide.
Further research is needed to elucidate on the potential role vitamin D plays in suicide.