Background: Suicide has been attributed to social and psychological factors but also to geophysical effects. Of the latter, changes in solar radiation and geomagnetic activities may contribute to the frequency and the seasonal pattern of suicides.
Methods: We studied with a population-based, nationwide analysis all the individuals who committed suicide (n=27,469) in Finland during the period of 1979 to 1999. The daily data on the number of suicides, and the mean and maximum levels of geomagnetic activity were compiled and modelled with Poisson regression using the number of inhabitants in each province as the denominator. Time series analysis of monthly numbers of suicides was carried out using a seasonal-trend decomposition procedure.
Results: There was a strong seasonal effect on suicide occurrence (P<0.00001), the risk of suicide being greatest in spring. The seasonal effect was most pronounced when the number of suicides was relatively low. High levels of solar radiation activity were associated with the increased risk of suicide (P=0.00001), but the effect of geomagnetic activity was weak.
Limitations: No individual data on alcohol consumption or mental disorders were available.
Conclusions: Suicide occurrence varies markedly by season and needs attention where prevention is concerned.