According to a recent article published in the Journal of the National Cancer Institute, more evidence mounts indicating that sleep interruption may significantly increase the risk of women developing breast cancer, particularly those who work night shifts.

Breast cancer is a common malignancy in the United States, with over 200,000 cases diagnosed each year. In response to the high incidence of this disease, researchers have focused efforts on screening and prevention strategies. Now, results from two separate clinical studies provide data indicating that women who work graveyard (night) shifts have a higher rate of breast cancer compared with women who work during the day.

The first study, conducted by researchers at the Fred Hutchinson Cancer Research Center, involved over 800 women who were diagnosed with breast cancer between 1992 and 1995. These women were interviewed in person to gather information on risk factor exposure in their lives including history of family cancer, reproductive history, alcohol consumption, cigarette smoking, sleeping habits, etc. Their answers were compared to women of the same age who did not have breast cancer.

The second study, conducted by researchers at the Brigham and Women’s Hospital and Harvard Medical School, involved nearly 80,000 women who answered questionnaires every 2 years. The questionnaire included the same types of questions as the interview questions in the first study.

Results from the first study indicated that graveyard shift work increased the risk of breast cancer by up to 60%. There was an increased risk with increasing years of working at night and with increasing hours per night shift. Among women who did not sleep between 1 and 2 a.m., there was a 14% increased risk of breast cancer for every night worked per week. In addition, there was an indication of an increased risk of among women with the brightest bedrooms.

Results from the second study also indicated an 8% increased risk in breast cancer in women who worked the night shift for up to 29 years and a 36% increase in breast cancer in women who worked the night shift for more than 30 years.

It is speculated that the hormone melatonin is somehow involved in the development of breast cancer in patients whose sleep is disturbed. Melatonin is released in the absence of light and is thought to be involved in the suppression of tumors through several biological processes. Melatonin is believed to directly suppress proliferation of cells, stimulate the immune system, scavenge free radicals, and modulate the expression of an important gene involved in cancer called the p53 gene. Laboratory studies have also indicated that impaired pineal secretion of melatonin is associated with an increased release of estrogen by the ovaries. Sleeping during the day or being exposed to light at night decreases the synthesis of and release of melatonin.

These results are important as they suggest an indirect association between melatonin and breast cancer. Women who work graveyard shifts should be especially diligent in screening procedures for breast cancer in order to catch the disease early when it is most curable. (Journal of the National Cancer Institute, Vol 93, No 20, pp 1513-1515, 2001)

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