October 20, 2014
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Breast Cancer

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Study: Sleep hormone linked to breast cancer

Are you afraid of the dark? Here's one reason not to be: getting a good night's sleep with the lights out could lower your risk of developing breast cancer.

Earlier research has shown that women who do late-night shift work may have as much as twice the risk of developing breast cancer as their 9-to-5 counterparts, though scientists have been in the dark as to why. But a study published in the Journal of the National Cancer Institute may shed some light on the issue.

Researchers at Brigham and Women's Hospital and Harvard Medical School in Boston hypothesized that this increased risk could have something to do with levels of melatonin, a hormone the body produces in the dark in order to promote sleep. Decreased levels of melatonin are associated with increased estrogen production, a known risk factor for breast cancer.

To test this theory, researchers used urine samples from women enrolled in the Harvard Nurses Study, in order to compare women's melatonin levels first thing in the morning. The Harvard Nurses Study tracked the health of almost 120,000 nurses since 1989, and as part of the research, more than 30,000 women have provided urine samples on a regular basis. In their research, the authors compared early-morning levels of the melatonin metabolite 6-sulphatoxymelatonin in the urine of 147 women who went on to develop breast cancer, and 291 "control" subjects who did not.

After analyzing the results and accounting for differences in sex hormone levels and other breast cancer risk factors, the researchers found that women who had the highest melatonin levels had a 41% lower risk of developing invasive breast cancer than women who had the lowest levels. The results were similar after the researchers re-analyzed the data excluding women with a history of working night shifts.

"These prospective data support the hypothesis that higher melatonin levels, as measured in first morning urine, are associated with a lower risk of breast cancer," the researchers wrote.

But they add that further studies are needed to confirm the findings, including whether the timing of sleep plays a role in breast cancer and whether melatonin supplements could play a role in its prevention.


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