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Tuesday, 16 October, 2001, 10:01 GMT 11:01 UK
Breast cancer link to nightshifts
Breast cancer cells
Breast cancer claims the lives of 10,000 British women each year
New evidence has emerged to support claims that women who work nightshifts are more likely to develop breast cancer.

Research carried out in America found that women who regularly worked night shifts for up to three years were 40% more likely to develop breast cancer than those who did not work at night.


The reported increase in risk is small and women who work nightshifts should not be unduly alarmed

Cancer Research Campaign
And the risk increased to 60% for those who regularly worked nightshifts for more than three years.

The results of the study, carried out in Seattle, suggest 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.

Danish study

The theory that nightshifts could be linked to an increased risk of developing breast cancer is not a new one but it has never been definitively proven.

In a study published last year, Danish scientists found that women who had worked predominantly at night for at least six months in their working life were 50% more likely to develop the disease.

The results of the latest study were based on the work history of 763 women with breast cancer and 741 without.

Scott Davis, an epidemiologist at the Fred Hutchinson Cancer Research Center in Seattle the latest study was carried out, said: "The numbers in our study are small, but they are statistically significant."

Changes in melatonin levels in men doing nightshifts may increase the risk of some types of male cancer, according to Dr Davis.

Biggest killer

Meanwhile, another study carried out at Brigham and Women's Hospital in Boston, found only a "moderately increased risk" of breast cancer after extended periods of nightshift work.

In that study the medical work histories of more than 78,000 nurses were studied over a 10-year period.

The results showed that nurses who worked rotating night shifts at least three times a month for one to 29 years were about 8% more likely to develop breast cancer.

For those who had worked night shifts for more than 30 years the relative risk of breast cancer went up by 36%.

A spokesperson for the Cancer Research Campaign said: "A number of studies have now hinted that night shift work could effect breast cancer risk.

"There is clearly a need for more research to clarify this effect and to compare it to known breast cancer risk factors like family history and obesity.

"The reported increase in risk is small and women who work nightshifts should not be unduly alarmed.

"Our advice to all women is to be breast aware, to attend for breast screening if you are between the ages of 50 and 64, and to report any changes to your doctor without delay."

Breast cancer is one of the biggest killers of women in the UK, claiming 10,000 lives a year.

See also:

10 Jan 01 | Health
Nightshift link to breast cancer
01 Oct 01 | Health
Cancer risk becomes clearer
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