Why do suicide rates peak in the spring?

Psychiatrists have been scratching their chins over this one for years. Counterintuitively, the arrival of spring and the long sunny days it ushers in, mark a staggering rise in suicide rates.

This week, mental health experts at the Priory group said that May is the peak month for suicides in Britain. "The increase can be dramatic, with up to 50% more successful suicides in some cases," says Chris Thompson, director of healthcare at the Priory group. In Britain, about 6,300 people take their own lives each year, 90% of whom are likely to have mental health problems.

The seasonal effect is seen all over the world, with the northern hemisphere witnessing a big rise in suicides in May and June and the southern hemisphere seeing a similar rise in November. While no one has a complete explanation as to why, the leading theory is that the increase is down to the effects of sunlight on our hormones.

According to Thompson, the seasonal changes that bring most of us out of winter apathy may work against those who are coming out of severe depression. "It is a harsh irony that the partial remission which most depression sufferers experience in the spring often provides the boost of energy required for executing a suicide plan," he says. "Spring is a time for new beginnings and new life, yet the juxtaposition between a literally blooming world and the barren inner life of the clinically depressed is often too much for them to bear."

Paradoxically, says Thompson, sunlight-driven changes in levels of the feelgood chemical serotonin may make people more aggressive and, if they are depressed, they could direct that aggression at themselves. The theory gains some support from research by Canadian scientists linking seasonal changes in bright sunlight with more violent suicides.

Other researchers believe that the influence of sunlight on another hormone, melatonin, is to blame. Sunlight inhibits production of melatonin, which is known to influence our behaviour.