The Journal of the National Cancer Institute published two studies that show that working at night increases a woman's risk for breast cancer. The first study from Seattle shows that women who go to work after seven in the evening have a 60 percent increase in breast cancer. The second, called the Harvard Nurse's Health study, shows that women who work at least three nights a month for 30 years have a 30 percent increase in breast cancer.

How can working at night cause breast cancer? When a woman on a conventional schedule goes to bed at night, she closes her eyes and the darkness allows her brain to produce increased amounts of melatonin, a potent antioxidant that helps prevent cancer. These women produce the most melatonin from one to two in the morning. On the other hand, women who work at night sleep in daylight hours, and the lack of darkness when they sleep prevents the brain from producing large amounts of melatonin. Lack of melatonin may cause breast cancer.

There are other studies that also associate lack of melatonin with breast cancer. People who sleep with the lights on at night are also at increased risk for breast cancer. Night time light shuts down melatonin. People who sleep near electric power lines also have lower brain levels of melatonin. Evidently melatonin decreases blood levels of estrogen when a woman sleeps. Women who work at night have lower blood levels of melatonin at night, which gives them higher estrogen levels at night, increasing stimulation of the breasts which can lead to cancer.

Journal of the National Cancer Institute, Novenmber, 2001

Reported 10/17/01; ghecked 9/3/05

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