Women who work at night may raise cancer risk

Women who work at night may be exposing themselves to an increased risk of breast cancer, say scientists who believe there could be a link to underproduction of melatonin, the brain's sleep chemical.

George Brainard, professor of neurology at the Thomas Jefferson University in Philadelphia, told the association there was mounting evidence to suggest that suppression of melatonin production in the body by exposure to bright, night-time light could be linked with breast cancer.

Melatonin is influenced by the intensity of light entering the eye and is a key component of the complex mechanism that controls the body's circadian clock, which it is why some travellers take it as a treatment for jet lag.

Professor Brainard said melatonin inhibited the growth of some cancer cells in the test-tube, and studies on animals had shown that suppressing melatonin by exposure to bright light at night increased the growth of tumours.

In humans, he said, epidemiological studies have shown that women who work night shifts over many years have an increased risk of breast cancer, and other researchers have shown a lower risk of breast cancer in blind women whose melatonin production is unaffected by night-time light.

Light can heal, and harm. It can help people suffering from seasonal affective disorder caused by long winter nights but it may also harm others because it induces an underproduction of melatonin, he said.

"In the long range, we think this will shape all artificial lighting, whether it's used for therapeutic purposes, or for normal illumination of workplaces, hospitals or homes," Professor Brainard said. "Broad changes in general architectural lighting may take years, but the groundwork has been established."

The exact mechanism that could implicate melatonin in suppressing breast cancer was still unresolved, he added.