Mark Rowe, The Independent, London
American scientists are to carry out urgent studies of a hormone
produced when we sleep, amid concerns that artificial illumination
at night may be a possible cause of breast cancer.
The scientists stressed this weekend 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 1 a.m.
and 3 a.m.
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 tumors of reproductive organs and the
liver. Melatonin is produced by the pineal gland in the brain and
secreted mostly at night between 9 p.m. and 8 a.m. 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 estrogen, 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
estrogen levels to reach a point where they can cause tumors 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 center, 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 tumors.
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.''
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
industrialized world is also being explored. ``We know that as
society has industrialized, breast cancer rates have gone up
dramatically,'' said 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 industrialized nations,
according to 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 estrogen 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 leukemia, 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 programs, 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.
