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Friday, 6 October, 2000, 13:16 GMT 14:16 UK
Cancer 'linked to' excess light
forth bridge at night
The UK's Forth Bridge - a beacon to mariners, a problem for astronomers
By Alex Kirby, BBC News Online environment correspondent and presenter of Costing the Earth

A US doctor claims exposure to high levels of nocturnal light may be linked to an increased risk of breast cancer.

He believes excess light disrupts basic body rhythms and may suppress the production of chemicals such as melatonin.

Richard Stevens, a cancer epidemiologist at the University of Connecticut health center, says his controversial theory is based on animal studies.

But Dr Timothy Key of the Imperial Cancer Research Fund, UK, told BBC News Online: "It's an interesting hypothesis, but as yet there is no direct evidence that this would be the case in humans."

Dr Stevens described his theory, based mainly on animal research, to BBC Radio 4's environment programme Costing the Earth.

He said: "The basic idea is that at night, when melatonin should be high, if there's light of sufficient intensity, this would suppress the melatonin.

Oestrogen increase

"So for example, if we go to the bathroom and turn on a bank of bright lights, that has the potential in most people to suppress melatonin.

floodlit malvern priory
Floodlights often waste energy
"Certainly in seasonally breeding animals, melatonin affects the female sex hormone, oestrogen. If the suppression of melatonin in humans increases oestrogen, as we think it may, chronically, then we know that elevated oestrogen increases the risk of breast cancer.

"This is not natural, in the sense that we did not evolve in an environment where this was possible. It is possible now, with our electric lighting."

Dr Stevens said US and Swedish scientists had found that blind women had a much lower risk of breast cancer - up to 50% lower.

He advised using a red light bulb at night, as that appears not to affect human melatonin production.

Another US researcher, Dr Graham Quinn, told the programme of his belief that excessive light may be associated with impaired sight.

Dr Quinn, an ophthalmologist at the Children's Hospital of Philadelphia, found that chickens kept in continuous light fed better than other birds, but also developed a lot of eye abnormalities.

Higher myopia levels

He gave a questionnaire to the parents of 479 children he treated, asking whether the children slept in the dark, with a night light, or with a room light.

He said: "We had 172 of the 479 children who slept in darkness, and 10% of those children were near-sighted.

"Of the children who slept with a small night light, 232 children, 34% had near-sightedness.

"And of the group that slept with a room light on, 75 children, 55% had myopia or high myopia."

light skies over poole
Darkness is hard to find
Dr Quinn said his findings were strictly provisional, and two later studies had failed to replicate his results. But he believed his theory warranted further testing.

Another contributor to the programme is an astrophysicist, Dr Chris Baddiley, who lives in the United Kingdom, and finds the brightness of the night sky makes his work increasingly difficult.

On a hillside in the English Midlands, he told Costing the Earth: "If it weren't for the pall of light, we'd see a dark sky with many, many stars, up to a thousand of them.

Depriving the young

"But I can't even identify constellations below about 30 degrees. Nobody can see the stars.

"It is young people who are switched on to science and technology quite often by seeing the glories of the Milky Way on a dark clear night. And it's just not happening any more."

Dr Baddiley said the amount of electricity wasted in the UK by lights which pointed straight up into the sky roughly equalled the continuous output of two power stations.

Costing the Earth was broadcast on BBC Radio Four at 2100 BST on 5 October.

milky way
The Milky Way as it is seldom seen

Photographs courtesy of Dr Chris Baddiley

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