Does working the night shift cause breast cancer?

Working the night shift is no one's idea of fun, but it turns out that it might be really bad for your health, too. Women who work at night are at a much greater risk of developing breast cancer, according to a new study by Danish researchers in the journal Occupational and Environmental Medicine. The findings have raised new alarm bells about shift work, which the World Health Organization's International Agency for Research on Cancer has listed as a "probably carcinogenic" activity. Here, a guide to the latest findings:

What does the study say?

The study, based on a survey of women in the Danish military, found that women who worked at least three night shifts a week over six years were more than twice as likely to develop breast cancer as those that didn't. Furthermore, women on the night shift who considered themselves morning people (or "larks") were four times more likely to be diagnosed with breast cancer.

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How is the night shift linked to cancer?

The study's authors hypothesize that the night shift disrupts the body's circadian rhythm, which affects levels of melatonin, a hormone that is believed to have healing, antioxidant properties. Melatonin levels normally increase during the darkness of night, but the artificial light of a night shift suppresses them.

What if women work fewer night shifts?

The risk of cancer subsides. Women who worked only one or two night shifts a week showed no increased risk of cancer. But environment isn't the only factor — the fact that larks are more vulnerable shows that those genetically disposed toward the morning undergo more debilitating disruptions when they work at night.

Have similar studies been conducted before?

Yes. Health organizations have recently cast a spotlight on health risks associated with disrupting circadian rhythms. In addition, scientists have studied the links between breast cancer and night shifts before, but they largely focused on nurses, whose increased incidences of breast cancer could be explained by greater exposure to radiation and other hospital-related treatments.

How authoritative is this study?

Third-party scientists say more work is needed to definitively establish a link between breast cancer and night shifts. While some say it's probable that a link exists, the study didn't weigh two of the biggest factors known to contribute to breast cancer: Diet and physical activity.

Sources: The Guardian, The Telegraph, TIME, UPI