The scientists yesterday stressed that their work is at an early stage and that women should not panic. Their starting point is research showing that blind women have a significantly lower risk of developing breast cancer than fully sighted women. They have also been struck by findings that breast cancer rates are five times higher in nations where artificial light has led to the creation of the "24-hour" society than in developing countries, where electric light is less pervasive.
The scientists are to investigate the effect that a combination of artificial light - and the lack of complete darkness this creates - may play in the suppression of melatonin. Natural light is not thought to be the problem because it does not occur at the time when melatonin production is believed to peak, between 1am and 3am.
The research will explore a possible link between melatonin, a hormone which is a key component of the body's sleep-wake cycle, and breast cancer, as well as tumours of reproductive organs and the liver. Melatonin is produced by the pineal gland in the brain and secreted mostly at night between 9pm and 8am. Melatonin production can be affected by shift work, insomnia, jet-lag and possibly even street lights filtering through curtains.
Scientists at Thomas Jefferson University in Philadelphia and the University of Connecticut now suggest melatonin may have a role in breast cancer because it may influence the amount of oestrogen, a hormone that might spur the growth of breast cancer, in the body. They now wonder whether the disruption of melatonin production - by the amount of artificial light we are exposed to - is allowing oestrogen levels to reach a point where they can cause tumours in the breast. There is as yet no direct proof of such a link and doctors have only the circumstantial evidence of the trials involving blind women augmented by studies with mice and rats.
The research into blind women found a proportionate decrease in breast cancer that coincided with the severity of the women's visual impairment. Dr Richard Stevens, a cancer epidemiologist at the University of Connecticut health centre, and colleagues in Finland found that women who are completely blind had 60 per cent fewer breast cancers than women with normal sight, according to findings published in the Journal of Cancer in June. Those women who were only slightly blind had similar breast cancer rates to fully sighted women.
Further studies of mice which have been exposed constantly to light or had their pineal glands removed - and so cannot produce melatonin - have shown that they can suffer higher rates of breast cancer. Rats that had their pineal glands removed but received melatonin supplements developed fewer tumours. Professor Charles Czeisler of Harvard University medical school is in no doubt about the disruptive role light can play in human lives. Writing in a recent edition of the US journal Science he said: "If light was a drug the government wouldn't approve it."
Dr Stevens believes the findings are not absolute proof of a link but make further research essential. "Women who are profoundly blind should be at lower risk of breast cancer because they can't perceive light at night," he said. "But we don't know if that's because their melatonin levels are less likely to be disrupted."
But the fact that breast cancer is a disease of the industrialised world is also being explored. "We know that as society has industrialised, breast cancer rates have gone up dramatically," said Dr Stevens. "The rates in the US and Western Europe are five times higher than in developing countries. We can explain less than half the number of breast cancers by known risk factors such as late age for your first-born child and the number of children you have but mostly we don't know why the rate is so high. There is a long way to go before we can say melatonin levels are implicated in breast cancer but you might tell the public to get a dark night's sleep."
Light and darkness play a crucial role in human health, according to Dr George Brainard, professor of neurology at Thomas Jefferson University in Philadelphia, who has studied the biological effects of light on humans for the past 15 years. "We've evolved through thousands of generations with the rising and setting of the sun, but in the past 100 years the electric light has affected our biological rhythms and had an acute effect on melatonin production," he said.
Even if a link is established between lower levels of melatonin and breast cancer, it may prove to be only one of several risk factors explaining why rates are higher in industrialised nations, according to Dr Brainard. "Breast cancer is likely to be a multi-risk issue where you may have several factors which may increase the chances of getting breast cancer," he said.
Kate Law, head of clinical information with the Cancer Research Campaign, said the role of melatonin should be investigated but said the trials of blind women did not prove a link. "We can't make that scientific jump," she said. "The trials did not examine whether or not these women had any of the major risk factors we associate with breast cancer. We also need to prove that melatonin is having a significant impact on oestrogen levels."
The disruption of melatonin production has also been raised as a possible explanation for the higher risk of breast cancer faced by air stewardesses. Finnish researchers reported four years ago that air stewardesses and female frequent fliers were found to have double the risk of breast cancer.
At first, the scientists suggested the cancer could be triggered by exposure to higher than average levels of radiation at high altitude. But leukaemia, the prime cancer associated with radiation, was not as common among the women as would be expected. In a letter to the Lancet medical journal last year, Dr Anthony Mawson, from Charlotte, North Carolina, suggested that jet-lag could be to blame. Because stewardesses can expect their sleeping cycles to be disturbed on long-distance flights, he said, it was possible they were producing less melatonin.
In Britain, around 10 per cent of women can expect to develop breast cancer in their lifetime, though the majority of these cases happen in later life. Survival from breast and cervical cancer is affected by the quality of screening programmes, how soon women consult their doctor with symptoms and the quality of treatment.
However, women are better at surviving cancer than men, according to a study commissioned by the Cancer Research Campaign and carried out by researchers from the Office of National Statistics and the London School of Hygiene and Tropical Medicine. The study, published earlier this year, showed that across most of the main cancers, women do better than men and, in some cases, significantly better.
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