Studies dating back to the 1800s have found that suicides peak in the spring and are lowest in winter. “If we take winter as a baseline, then there’s a 20-60% higher suicide rate during spring,” says Fotis Papadopoulos, a professor of psychiatry at Uppsala University in Sweden, who has been studying the association.

This seems rather counterintuitive considering that darker days are linked to low mood. How could this be?

One possibility is that this is a result of changing levels of serotonin – a neurotransmitter that regulates mood – within the brain. Studies have found that serotonin levels in the blood are higher during the summer than the winter, and that there’s a positive correlation between serotonin synthesis and the hours of sunshine on the day that a blood sample is taken.

There’s also a further connection, in that selective serotonin reuptake inhibitor (SSRI) antidepressants, which boost serotonin, have been linked to risk of suicide in a small number of patients. “We know that when we treat patients with antidepressants it can take at least three or four weeks to raise their mood,” says Papadopoulos. “During this time, some people become more physically active or agitated, which could potentially make them more likely to act on their thoughts. Maybe sunshine acts in a similar way in a minority of people.”