Relationship between daylength and suicide in Finland

Laura Hiltunen,Kirsi Suominen,Timo Partonen,Jouko Lönnqvist

doi:10.1186/1740-3391-9-10

Laura Hiltunen, Kirsi Suominen + Show 2 more

Open Access

https://doi.org/10.1186/1740-3391-9-10

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Abstract

BackgroundMany previous studies have documented seasonal variation in suicides globally. We re-assessed the seasonal variation of suicides in Finland and tried to relate it to the seasonal variation in daylength and ambient temperature and in the discrepancy between local time and solar time.MethodsThe daily data of all suicides from 1969 to 2003 in Finland (N = 43,393) were available. The calendar year was divided into twelve periods according to the length of daylight and the routinely changing time difference between sun time and official time. The daily mean of suicide mortality was calculated for each of these periods and the 95% confidence intervals of the daily means were used to evaluate the statistical significance of the means. In addition, daily changes in sunshine hours and mean temperature were compared to the daily means of suicide mortality in two locations during these afore mentioned periods.ResultsA significant peak of the daily mean value of suicide mortality occurred in Finland between May 15th and July 25th, a period that lies symmetrically around the solstice. Concerning the suicide mortality among men in the northern location (Oulu), the peak was postponed as compared with the southern location (Helsinki). The daily variation in temperature or in sunshine did not have significant association with suicide mortality in these two locations.ConclusionsThe period with the longest length of the day associated with the increased suicide mortality. Furthermore, since the peak of suicide mortality seems to manifest later during the year in the north, some other physical or biological signals, besides the variation in daylight, may be involved. In order to have novel means for suicide prevention, the assessment of susceptibility to the circadian misalignment might help.

Highlights

Many previous studies have documented seasonal variation in suicides globally
Based on our earlier psychological autopsy studies of death from suicide [22] and the data from the nationwide suicide program in Finland [23], we hypothesized that the circadian misalignment among the depressed may increase during spring, and thereby predispose to suicidal behaviors [24]
To evaluate the effect of daily sunshine hours and temperature on suicide mortality, we focused on two cities on a similar longitude but with dissimilar photoperiod: first, Helsinki (60°9.7’N, 24°57.3’E), which is the capital of Finland in the south, and second, Oulu (65° 1.0’N, 25°30.0’E), which is a central city of the northern part of the country, 600 km north from Helsinki

Summary

Introduction

Many previous studies have documented seasonal variation in suicides globally. We re-assessed the seasonal variation of suicides in Finland and tried to relate it to the seasonal variation in daylength and ambient temperature and in the discrepancy between local time and solar time. There is a clear peak of suicide occurrence around May or June [4,5,6,7] and a preceding peak in suicide attempts around April [8]. Another smaller peak of suicide occurrence exists around October [7,9]. Based on our earlier psychological autopsy studies of death from suicide [22] and the data from the nationwide suicide program in Finland [23], we hypothesized that the circadian misalignment among the depressed may increase during spring, and thereby predispose to suicidal behaviors [24]

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