Research report
Cyclic time patterns of death from suicide in northern Finland

https://doi.org/10.1016/S0165-0327(02)00236-7Get rights and content

Abstract

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.

Introduction

Suicides are not evenly distributed over particular periods of time, since social or psychological factors have immediate or delayed effect on the individual’s decision to commit suicide (Mann, 1998). Most studies have found that suicide rates tend to peak during spring and summer Yip et al., 1998, Altamura et al., 1999. Moreover, it appears that climate and weather conditions can somewhat influence the occurrence of death from suicide, possibly by interacting with social factors Preti, 1997, Salib and Gray, 1997, Preti and Miotto, 1998.

Physical time-givers, as well as social and endogenous ones, can modify complex behaviours. Seasonal transition may contribute to the pathogenesis of the suicide process due to the substantial changes in day length and temperature that occur in spring and autumn. The asynchrony between changes in temperature and light–dark cycles challenges the stability of endogenous clocks (Czeisler et al., 1999). It may acutely result in disordered periods of activity (Taillard et al., 2001), or disorganised behaviour, possibly leading to suicide in adverse circumstances.

Since geophysical factors may markedly influence daily behaviour patterns, it is important in terms of prevention to study whether they can also affect the occurrence of death from suicide. We therefore set out to study the time series of death from suicide in the population of a far northern region where there is extensive variation in levels of ambient temperature and daylight over the year. We hypothesised that increasing light exposure on still cold days during spring in specific may drive the individual into a later activity phase and the failure to adjust to time-giving signals from the natural habitat during this period of seasonal mismatch may lead to death from suicide more frequently than by chance alone.

Section snippets

Methods

The study material consisted of all suicides (n=1658) committed from 1 April 1987 to 31 December 1999 in the province of Oulu, which is located 64–66°N in northern Finland, a northern European country with approximately 5 million inhabitants. Date, sex and age were recorded for each case. Population data on the number of inhabitants, sex ratio and age groups in the province during the study period were derived from statistical yearbooks published by Statistics Finland (1988–2000).

Cause of death

Results

Of the 1413 male and 313 female suicides there were complete data available for analysis on 1658 (96%) subjects (Table 1). Male sex (χ2=735.3, df=1, P<0.0000001) and two older age groups (30–60, plus over 60 years, χ2=344.7, df=2, P<0.0000001) were associated with the risk of suicide. Our analyses showed that marked changes in the number of suicides occurred during the study period (χ2=28.0, df=12, P=0.01). The risk of suicide decreased significantly over the three consecutive years from 1994

Discussion

Our main finding was that contrary to recently published findings Yip et al., 2000, Parker et al., 2001 there was a significant seasonal variation in death from suicide. The highest incidence was observed at the turn of summer to autumn, and a second peak occurred during spring. Our findings also suggest that despite a marked difference in time patterns for suicide between men and women in the long term, there is none in the seasonal variation.

The seasonal effect was modest though, as compared

Conclusion

Marked changes in the number of deaths from suicide were observed during the study period. There was a substantial seasonal variation in suicide occurrence among the study population. Compared to sex and age, however, the seasonal effect on the risk of suicide remained modest. Periods of seasonal mismatch, local weather conditions, and geomagnetic storms had no effect on the risk.

Acknowledgments

We thank Heikki Nevanlinna, PhD, from the Finnish Meteorological Institute, for help with the interpretation of geomagnetic data, and Markus Henriksson, MD, from the National Public Health Institute, for intellectual support.

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