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Night light link to breast cancer

20th February 2008

Women living in neighborhoods with large amounts of nighttime illumination are more likely to get breast cancer than those who live in darker areas, a study that overlaid satellite images of Earth onto cancer registries showed.

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The finding bolsters the hypothesis that exposure to too much light at night can raise the risk of breast cancer by interfering with the brain's production of a tumor-suppressing hormone.

"By no means are we saying that light at night is the only or the major risk factor for breast cancer," The Washington Post quoted Itai Kloog, of the University of Haifa in Israel, who led the new work, as saying. "But we found a clear and strong correlation that should be taken into consideration."

Scientists have known for years that rats raised in cages where lights are left on for much of the night have higher cancer rates than those allowed to sleep in darkness. Studies of nurses, flight attendants, and others who work at night have found breast cancer rates 60% above normal, even after controlling for other factors.

The World Health Organisation announced in December its decision to classify shift work as a "probable carcinogen," putting night shifts in the same health-risk category as exposure to such toxic chemicals as trichloroethylene, vinyl chloride, and polychlorinated biphenyls (PCBs).

The mechanism of such a link, if real, remains mysterious, but many scientists suspect that melatonin is key. Secreted by the pineal gland in the brain, the hormone helps prevent tumor formation. The body produces melatonin primarily at night, and levels drop precipitously in the presence of light, especially light in the blue part of the spectrum produced in quantity by computer screens and fluorescent bulbs.

 

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