LONDON (Reuters) - Too much sunlight in places like Greenland where long summer days often cause insomnia appears more likely to drive a person to suicide, Swedish researchers said Friday.

Despite a belief that suicides tend to rise in late autumn and early winter months because of darkness, the new findings suggest that places where constant sunlight in summer seasons is a fact of life may be just as dangerous.

"During the long periods of constant light, it is crucial to keep some circadian rhythm to get enough sleep and sustain mental health," Karin Sparring Bjorksten of the Karolinska Institute in Sweden and colleagues reported in the BioMed Central journal BMC Psychiatry.

According to the World Health Organization, 877,000 people worldwide kill themselves each year. For every suicide death, anywhere from 10 to 40 attempts are made, the U.N. agency estimates.

Scientists have previously linked sleep disturbances to increased suicidal risk in people with psychiatric disorders and in adolescents but it is unclear whether the association also exists in the general population.

The Swedish team studied the seasonal variation of suicides in all of Greenland from 1968 to 2002 and found a cluster of suicides in the summer months.

This seasonal effect was especially pronounced in the north of the country -- an area where the sun doesn't set between the end of April and the end of August.

"We found that suicides were almost exclusively violent and increased during periods of constant day," Bjorksten said in a statement.

"In the north of the country, 82 percent of the suicides occurred during the daylight months."

Most of the suicides involved young men and were violent -- such as shooting, hanging and jumping from high places. These kinds of deaths accounted for nearly all, about 95 percent, of the suicides.

The researchers speculated that light-generated imbalances in serotonin -- the brain chemical linked to mood --may lead to increased impulsiveness that in combination with a lack of sleep drives people to kill themselves.

"Light is just one of the many factors in the complex tragedy of suicide, but this study shows that there is a possible relationship between the two," Bjorksten said.

(Reporting by Michael Kahn; Editing by Maggie Fox)