Workers who regularly have the night shift may be at increased risk of breast cancer, according to researchers from Thomas Jefferson University in Philadelphia.

The researchers theorize that repeated late nights under electric lights can disrupt a woman’s hormonal balance by depleting her supplies of melatonin, a brain hormone. A shortage of melatonin may increase the supply of estrogen, boosting the risk of breast cancer.

Studies of female nurses indicated that those working at least three night shifts a month are almost 40 per cent more likely to develop breast cancer than those who work during the day, according to the researchers.

"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 said at a conference of the American Association for the Advancement of Science in Denver.

"In industrialised Western societies, breast cancer rates are extraordinarily high. One in every seven women in nations such as ours will get breast cancer," Brainard said. In developing countries, which do not have a prevalence of electric light, breast cancer rates are five times lower, he added.

Other sources: American Association for the Advancement of Science, BBC