Does diurnal temperature range influence seasonal suicide mortality? Assessment of daily data of the Helsinki metropolitan area from 1973 to 2010.

Several studies show a peak in suicide rates during springtime and suggest differences in the seasonal variation of suicides. However, the seasonal distribution of the temperature impact on suicide is less clear. This study investigated the relationship between diurnal temperature range (DTR) on suicide mortality. Daily temperature and suicide data for Helsinki were analyzed for the period of 1973-2010 inclusive. Overall, DTR reached its maximum during the spring from mid-April to mid-June, which is also the season with highest suicide mortality in the study region. Specifically, the seasonal timing and maxima for both DTR and suicides vary from year to year. Time series analysis of DTR and suicide records revealed a significant (P<0.01) correlation between the springtime DTR maxima and suicide rates for males. No similar association could be found for females. These results provide evidence that a higher springtime DTR could be linked statistically to a higher seasonal suicide rate each spring, whereas the exact timing of the DTR peak did not associate with the seasonal suicide rate. A possible mechanism behind the springtime association between the DTR and suicides originates from brown adipose tissue (BAT) over-activity. Activation of BAT through the winter improves cold tolerance at the cost of heat tolerance. This might trigger anxiety and psychomotor agitation, affecting mood in a negative way. As a hypothesis, the compromised heat tolerance is suggested to increase the risk of death from suicide.