Women who work night shifts have an increased risk of developing breast cancer, according to doctors at the Danish Cancer Society in Copenhagen.
Doctors Johnni Hansen and Christina F. Lassen examined data on nearly 700 women who worked for the Danish army in the past five decades, including 141 who had survived breast cancer, and calculated that night shift work was associated with a 40% increased risk of developing the disease.
The more frequent and prolonged the night shift work, the greater the risk: Women who worked nights at least three times a week for six years or more were more than twice as likely to develop the disease than those who did not, the study found. But working two night shifts a week or less had no impact.
While a lack of sunlight has been linked to various cancers, so has too much sun exposure, and the Danish researchers found that night shift workers tended to sunbathe more frequently than day workers.
The Danish study appears in Occupational and Environmental Medicine, published by the British Medical Journal.
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