Some places north of the Arctic Circle have months of constant darkness in winter and permanent daylight in summer. One might think that people would be more likely to kill themselves in the darker months but a study of suicides in Greenland by researchers at the Karolinska Institute in Sweden has found that suicides tend to cluster in the summer months. In northern Greenland - where the sun doesn't set between the end of April and the end of August - 82% of suicides occured in summer. Most of these suicides involved young men and were violent including deaths by shooting, hanging and jumping from high places. The researchers thought that increased serotonin, caused by the extra sunshine, could make people more impulsive while insomnia caused by the constant daylight made people more suicidal.
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