Women who work night shifts are at higher risk of breast cancer, particularly those women who don't consider themselves "night owls," say researchers.


A new study, funded by the Danish Cancer Society, involved more than 18,500 women working for the Danish army between 1964 and 1999. Of these women, 210 had breast cancer. These women were compared with 899 women of the same age who had not developed breast cancer. The women in the study completed a detailed questionnaire regarding their working patterns, use of the birth control and hormone replacement therapy, sunbathing habits and whether they classified themselves as a "morning" or "night" person.  


The research suggests working at night increases the chances of the developing breast cancer by 40 percent. Women working more than two night shifts a week have double the risk of those who work day shifts. The effect was even greater for women working the night shift who describe themselves as "morning people" rather than "night owls."


It's unclear why night shift workers might be at greater risk for developing breast cancer, but one theory is that a hormone in the body that potentially suppresses tumors, melatonin, may be disrupted by constant exposure to light during night hours.


Night shift increases breast cancer risk
New research finds that a woman who work at least 2 night shifts a week may double her breast cancer risk.