Night shifts 'increase breast cancer risk'

Night work may alter the body's chemical balance

Scientists have produced more evidence that night work can damage health.

An analysis of data from Denmark suggests women who work at night may be at an increased risk of developing breast cancer.

The study follows research which suggests working at night increases the risk of heart disease.

This study is big enough not to be swept under the carpet

Professor Gordon McVie, Cancer Research Campaign

Dr Johnni Hansen, of the Institute of Cancer Epidemiology in Copenhagen, studied the medical and employment records of 7,035 women between the ages of 30 and 54 who had been diagnosed with breast cancer.

He found that women whose work involved night shifts were 1.5 times more likely than daytime workers to be diagnosed with breast cancer.

Dr Hansen found that the longer a woman regularly worked at night, the higher her risk of developing the disease.

The reason for the link is unclear.

But some studies have suggested night workers are more likely to consume alcohol, a known risk factor for breast cancer.

Artificial light

Another theory is that the risk is increased by exposure to artificial light.

It is thought artificial light acts to suppress production of a hormone called melatonin, which is normally produced in the body at night.

Research has suggested low levels of melatonin may either stimulate the growth of cancerous cells in the breast, or encourage the production of the female sex hormone oestrogen, which has been widely linked to breast cancer.

Dr Hansen argues that the experience of blind women provides evidence to back this theory up.

Blind women have approximately a 50% smaller risk of developing breast cancer than other women.

As these women cannot sense light visually, it has no impact on their melatonin production.

Professor Gordon McVie, director general of the Cancer Research Campaign, said: "This study is completely inconclusive, and the increased risk is not enormous.

"But it is a big enough study not to be swept under the carpet."

Prof McVie said the fact that people who live in the Mediterranean area do not suffer higher rates of breast cancer suggested the melatonin theory may not be correct.

He said several other hormones could equally be implicated in any possible link between night work and breast cancer

The research is published in the journal Epidemiology.