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Since the world does not stop at five in the evening, many people are happy to take the late shift. Unfortunately, the night shift also is linked with higher rates of breast cancer, and the International Agency for Research on Cancer have even gone so far as to call the night shift a cancer-causing agent. A recent study published by researchers from the University of Cologne in Germany has found that working the night shift is linked to a significantly higher rate of ovarian cancer.

The study was conducted with 3,322 women in total. Among the total group of women, 1,101 women had the most common type of advanced ovarian cancer, 389 had a borderline disease and 1,832 women served as healthy controls. All of the women were between the ages of 35 and 74, and each was asked about the hours they worked, particularly on whether they had over worked the night shift.

Thomas C. Erren, the study's researcher, found that, across all groups, the women worked in night shifts for an average of 2.7 to 3.5 years. These jobs were diverse, varying among healthcare, food preparation and service, as well as office and administration support. Women who were 50 years old or older were significantly more likely to have ovarian cancer if they had worked the night shift.

Among the three groups, 26.6 percent of the women with advanced invasive cancer had worked the night shift, 32.4 percent of the women with early stage cancer had worked nights and 22.5 percent of the women who did not have ovarian cancer had worked at night. In total, working the night shift was linked with a 24 percent elevated risk of advanced ovarian cancer, while it was linked with a 49 percent increased risk of the early stage disease.

Women with ovarian cancer were less likely to have taken the birth control pill and tended to have fewer children. The Pill and motherhood are associated with decreasing the risk for ovarian cancer.

Interestingly, Erren, the study author, found that the link for ovarian cancer may be weakened for night owls. While 20 percent of people described themselves as morning people, 29 percent of people said they were night owls. Early birds were both more likely to have advanced cancer and borderline tumors, though the difference was not statistically significant.

It is believed that the link between the night shift and ovarian cancer may be due to a hormone called melatonin. The hormone, which regulates reproductive hormones like estrogen, scavenges the body for free radicals and increases the creation of antioxidants, is normally produced at night, but is suppressed with surrounding light.

The study was published in the journal Occupational and Environmental Medicine.