Cyclic time patterns of death from suicide in northern Finland

BACKGROUND: Time patterns of suicide have been attributed not only to social and psychological factors but also to direct geophysical effects. Seasonal variations in day length and temperature seem likely to contribute to the timing of the suicide process. METHODS: We analysed all suicides (n=1658) committed in a northern province of Finland during a period of 153 months. Daily data on the number of suicides, local weather conditions and geomagnetic storms were compiled and modelled with Poisson regression using the province population as the denominator, and with the means of harmonic series for seasonal variation. Time series analysis of monthly numbers of suicides was carried out using the seasonal-trend decomposition procedure based on loess. RESULTS: Marked fluctuations in the number of suicides occurred during the study period (P=0.01). There was significant seasonal variation in death from suicide (P=0.01), but analysis of the meteorological data showed no evidence of effect on the risk of suicide. LIMITATIONS: Assessment of mental disorder or alcohol consumption was missing, since only data derived from death certificate was available for each case. CONCLUSIONS: The seasonal effect was significant, but remained modest compared to sex and age as risk factors for suicide. Preventive measures need to be tailored according to time of the year.