Researchers at a science conference in the US have produced more evidence to show how night-time activity may increase the incidence of breast cancer.

Their findings presented to the meeting of the American Association for the Advancement of Science in Denver suggest shift-workers may be particularly at risk from the disease.

It adds further to the evidence already gathered on the subject by other researchers, particularly with data from Denmark.

Theory suggests that bright light at night diminishes the body's supply of melatonin - a hormone involved in the control of the body's natural rhythms.

This in turn may lead to an increase in levels of the female sex hormone oestrogen, which has been linked to breast cancer.

Researchers are looking into whether risks can be reduced by using different kinds of lighting.

'Serious'

"Our work is showing that light at night may be a risk for breast cancer. That is a very serious problem for industrialised countries," Dr George Brainard of Thomas Jefferson University in Philadelphia told the BBC.

Breast cancer affects 'one in seven women'

"In industrialised Western societies, breast cancer rates are extraordinarily high. One in every seven women in nations such as ours will get breast cancer."

In developing countries, which do not have a prevalence of electric light breast cancer, rates among women are five times lower, he said.

He said scientists were wondering whether light was a factor determining why blind women appeared to have a lower risk of breast cancer.

Meanwhile women who travel over many time zones are more likely to get the disease.