Danish Study Suggests Night Shift Work May Increase Breast Cancer Risk


A new study was published in the May 28 online edition of Occupational and Environmental Medicine that reports on the possible effects of night shift work on the incidence of breast cancer.

The study also looked at breast cancer risk differences in morning vs evening people.

A Danish study found that women who work the night shift more than twice a week might be increasing their risk for breast cancer, The risk appears to be cumulative and highest among women who describe themselves as “morning” people rather than “evening” people.

Johnni Hansen, of the Institute of Cancer Epidemiology at the Danish Cancer Society, in Copenhagen and lead researcher of the study, puts the number of women currently performing night shift work at between 10 to 20 percent. He shared that night shift work might be one of the largest occupational problems related to cancer.

Hansen’s team collected data on more than 18,500 women who worked for the Danish Army between 1964 and 1999 for their study.They identified 210 women who had breast cancer and compared them to almost 900 similar women who did not have breast cancer.

Women responded to questions about their working patterns, lifestyles habits and factors including contraceptive use, hormone replacement therapy, and sunbathing.

Women were also asked to identify themselves as “morning” or “evening” persons.

Respondents included 141 women with breast cancer, and 551 women who did not have breast cancer.

Findings showed that the risk for breast cancer was increased 40 percent if they worked at night, For women who worked nights at least three times a week, and for at least six years, the risk was doubled.

Women who worked the night shift but who described themselves as morning people were at even higher risk of breast cancer. They were almost four times more likely to develop breast cancer as those who didn’t work nights.

Additionally, Hansen’s team found that women who considered themselves evening people were twice as likely to develop breast cancer. Morning-preferring women who did not work at night had a lower overall risk of breast cancer than evening types.

At the moment, possible reasons for the study findings are not certain. However, Hansen offers factors that may be contributing to increased risks that are particular to night shift work such as:

“Night shift work involves exposure to light at night, which decreases the production of the night hormone melatonin that seems to protect against certain cancers. In addition, light at night might introduce circadian disruption, where the master clock in the brain becomes desynchronized from local cellular clocks in different body organs, affecting the breast.”

Hanson continued saying, “Repeated phase shifting may lead to defects in the regulation of the circadian cell cycle, thus favoring uncontrolled growth. Also, sleep deprivation after night shift work leads to the suppression of the immune system, which might increase the growth of cancer cells.

Since night shift work is unavoidable in modern societies, this type of work should be limited in duration and limited to less than three night shifts per week,” Hansen concluded.

While the study found an association between night shift work and breast cancer, it did not prove a cause-and-effect relationship.

(SOURCES: May 28, 2012, Occupational and Environmental Medicine, online )


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