Burning the midnight oil could lead to breast cancer

Sleeping with the light on or staying up late could cause breast cancer, authoritative new research suggests.

The research - being hailed as a "watershed" that provides "the first proof" of a link between artificial light at night and cancer - confirms a mass of the studies suggesting that modern life causes the disease by interfering with natural sleep cycles.

Carried out by the National Cancer Institute and National Institute of Environmental Health Sciences in the United States, it offers a solution to the mystery of rising levels of breast cancer in rich countries, which are five times as high as in the developing world. One in 10 women will develop the disease.

Experts believe that half the cancers are likely to be accounted for by family history, smoking, drinking alcohol, diet, medicines, and such reproductive factors as childlessness and having children late. But evidence has been building that electric light during the hours of darkness may be responsible for much of the rest.

Repeated studies have shown that night-shift workers - such as nurses or air stewardesses - are up to 60 per cent more likely to get the disease. Another found that women who stayed up late two or three times a week were similarly susceptible. Conversely, totally blind women were half as likely to succumb to it.

"If light were a drug, the Government would not approve it," said Professor Charles Czeisler of the Harvard Medical School.

Professor George Brainard of Thomas Jefferson University in Philadelphia, added: "Humans evolved on a planet without electric light over thousands and thousands of generations. The body is designed to be alert and awake during daytime hours and to sleep at night. Now we have a 24/7 society that isn't in harmony with our biological design."

Studies show that light at night interferes with one of the body's greatest natural defences against cancer - melatonin, dubbed "the hormone of darkness".

The hormone - which the pineal gland secrets at night, particularly in the early hours of the morning - impedes the growth of cancers and boosts the immune system.

Light, however, stops its production, making the body think it is daytime.

In the new study, scientists grafted human breast cancer tumours on to rats and infused them with blood taken from women during the day, in the early hours of the morning, and after being exposed to light at night.

The blood taken in darkness slowed the growth of the cancers by 80 per cent, but the blood taken after exposure to light accelerated it.

Professor Richard Stevens of the University of Connecticut, who described it as "a watershed study", said: "Electric light as a driver of the breast-cancer epidemic worldwide - that's a dramatic big thing."

And Dr David Blask, who led the research - and called it "the first proof that light is indeed a risk factor for cancer" - added: "Breast tumours are awake during the day, and melatonin puts them to sleep at night." Add artificial light and "cancer cells become insomniacs".

THE RISK

* Researchers have believed for some time that too much light during the night could lead to breast cancer.

* The reason is that light stops the production of melatonin, a hormone that fights cancer.

* The latest study found that cancers grew faster in rats injected with blood taken from women exposed to light at night.

- INDEPENDENT