NEW YORK, Mar 12 (Reuters Health) - High blood levels of the hormone melatonin may explain why blind women have significantly lowered risks for breast cancer compared with sighted women, researchers report.

Melatonin, which is secreted by the pineal gland in the brain and plays an important role in the body's sleep cycle, is thought to influence the secretion of estrogen, which in turn influences breast cancer risk.

"Our findings give support to the 'melatonin hypothesis,'" write Dr. J. Kliukiene and colleagues, of the Cancer Registry of Norway in Oslo.

Reporting in a recent issue of the British Journal of Cancer, the researchers used Norwegian government health data to track breast cancer in over 15,000 visually impaired women, nearly 400 of whom were totally blind.

The investigators found that totally blind women had a 36% lower risk of breast cancer compared with sighted women. Women who became blind relatively early in life (before age 65) appeared to be especially protected against breast cancer, with incidence rates 49% below those of sighted women.

Only total blindness--not visual impairment--seemed to protect against breast cancer. According to the authors, this supports the theory that increased nighttime exposure to artificial light reduces melatonin levels, altering estrogen secretion rates and upping risks for breast cancer.

Blind women are by definition unreceptive to light, however, and may maintain high melatonin production at night regardless of external light conditions. Kliukiene's team believe this may be the mechanism whereby blind women are protected from breast cancer.

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