Research reportSeasonal changes in 24-h patterns of suicide rates: a study on train suicides in The Netherlands
Introduction
Seasonality of suicidal behaviour has been an object of study since the previous century. The observed peaks in suicide rates near the summer solstice have been a matter of interest and debate for over 150 years (Kevan, 1980; Aschoff, 1981; Massing and Angermeyer, 1985). It has been suggested that variations of psychosocial stress over time of day and season may account for some of the temporal variations in suicide rates (Durkheim, 1897). More recently, the idea has been put forward of temporal variations in susceptibility to psychosocial stressors. This variation in susceptibility might be attributable to changes in central 5-HT (serotonin) neurotransmission (Maes et al., 1993). One argument in favour of this hypothesis is the observation of a significant temporal relationship between plasma l-tryptophan (a precursor of 5-HT) levels in healthy subjects and rates of violent suicide (Maes et al., 1995).
Whatever the mechanism by which the seasonal pattern of suicide rates is caused, there is a significant correlation between day length and suicide rate, and between hours of sunshine and suicide rate (Aschoff, 1981). If day length somehow would be causally involved in suicidal behaviour, it would be expected that the 24-h patterns of suicide rates would vary in the course of the year. Such circadian variations of suicidal behaviour (i.e., variations over the 24-h day) have received little attention. Uncertainty about the exact timing of many suicides complicates research in this field (Barraclough, 1976). Both Durkheim (1897) and Morselli (1882) noted that suicide is largely restricted to the daytime hours. These observations are confirmed by more recent major studies (Williams and Tansella, 1987; Maldonado and Kraus, 1991; O’Donnell and Farmer, 1994; Schmidtke, 1994) showing the lowest suicide rates between midnight and 06:00 h. However the temporal distribution outside this nocturnal period is unclear. Long intervals of three or more hours, chosen by most authors as the unit of temporal analysis, preclude a detailed analysis of 24-h patterns. Only two studies investigated the possible interaction between daily patterns of suicide rate and season (Schmidtke, 1994; Altamura et al., 1999). In the most recent study (Altamura et al., 1999) regarding different, but predominantly violent, types of suicide in Italy (39° N), no significant interactions were found between time of day of suicide and the quarters of the year. In the other study (Schmidtke, 1994), carried out in Germany (47°30′–54° N), the 24-h patterns of train suicide rates in the summer and winter season were compared. The peak of train suicides in males, was found to shift from 18:00–21:00 h in winter to 21:00–00:00 h in summer.
The present study focuses on train suicide as well. The advantage of investigating this type of suicide is that the exact moment of occurrence is known. Specific legal procedures connected with every train accident, i.e., collision with object, ensure detailed recording of circumstances and time. As a consequence train suicides are the best documented suicides in terms of time in The Netherlands and a number of other countries. In The Netherlands, train suicides accounted for 10–14% of all suicides in the period from 1980 to 1994. These percentages are higher than those reported in other European countries, like Germany and Austria, where they are 5.1 and 5.7 (Schmidtke, 1994; Deisenhammer et al., 1997), respectively. Still these percentages are small. Therefore it remains to be investigated whether the conclusions of the present study can be generalized to suicides in general.
Here, we specifically address the question of whether 24-h patterns of train suicide rates can be identified, and if so, whether they change with time of year. The results will be discussed in the context of possible environmental and biological factors that might be involved in processes underlying the suicidal behaviour under study.
Section snippets
Method
The Netherlands is a small, densely populated country with approximately 15 million inhabitants. It is located between 51 and 53°30′ N, in a temperate climate zone where the length of day varies considerably over the seasons. At this latitude sunrise varies from 08:46 h (Central European Time) in mid-winter to 05:18 h (Central European Summer Time) at the summer solstice. Sunset varies from 16:30 to 22:03 h. So the length of the day varies from 7 h and 44 min to 16 h and 45 min.
In the period
Results
The annual pattern of train suicides in The Netherlands, accumulated over 15 years, is presented in Fig. 1. Averaging the results and dividing this by 15 yields a mean of 14.5 cases per 4 weeks. Obviously, the data do not show the expected peak near the summer solstice. Poisson regression analysis indicates that there is no significant time trend across the months of the year (χ2=17; df=11; P=0.11).
When we analyse the same data relative to clock time, a pronounced 24-h pattern emerges (Fig. 2).
Discussion
In spite of the fact that the data on train suicides in The Netherlands, collected over 15 years, do not demonstrate a pronounced seasonal dependence when plotted as a function of time of year, nor show a pronounced diurnal pattern when plotted as a function of clock time, the data do reveal a strong and significant seasonal modulation of diurnal suicide patterns when plotted two-dimensionally. The diurnal distribution changes over the year in such a way, that peaks in summer occur at clock
Acknowledgments
We thank the N.V. Nederlandse Spoorwegen for kindly providing access to the data used in this study. We thank Professor Dr. R.H. van den Hoofdakker and Professor Dr. J.C. Gillin for commenting on a previous version of the manuscript. We thank Dr. J. Neeleman for his statistical advice. We are also grateful to A.M. Steensma for setting up the database and performing preliminary analyses.
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