The researchers identified 210 women who had breast cancer and compared them to almost 900 similar women who did not have breast cancer.
All of the women were asked about their working patterns, lifestyles and other factors such as their use of contraceptives and hormone replacement therapy, and their sunbathing habits.
In addition, women were asked to classify themselves as "morning" or "evening" people.
In all, 141 women with breast cancer responded to the study questionnaires. In addition, 551 women who did not have breast cancer responded.
Among these women, the risk for breast cancer was increased 40 percent if they worked at night, the researchers found.
But for women who worked nights at least three times a week, and for at least six years, the risk was doubled, the findings showed.
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, according to the researchers.
In comparison, women who considered themselves evening people were twice as likely to develop breast cancer, they added.
Morning-preferring women who did not work at night had a lower overall risk of breast cancer than evening types, Hansen's team found.
"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 said. "In particular, morning types should limit their night work," he added.
While the study found an association between night shift work and breast cancer, it did not prove a cause-and-effect relationship.
Men who work at night may also be at risk for prostate cancer, Hansen noted. This evidence comes from three small studies, he said.
Dr. Stephanie Bernik, chief o
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