Working the night shift: a risk factor for breast cancer?

by Mary Calvagna, MS

Breast cancer is the most commonly diagnosed female cancer in the United States and Europe (not including skin cancer). The cause of breast cancer is unknown, and only a small proportion of the women who get breast cancer have a known risk factor.

In many countries, the incidence of breast cancer is increasing. Science is struggling to explain why this may be and to answer many of the questions surrounding breast cancer. One avenue being explored is identifying risk factors. Two independent studies published in the Journal of the National Cancer Institute looked at exposure to light at night as a potential risk factor for breast cancer.

The first study involved 813 women who were diagnosed with breast cancer between November 1992 and March 1997. Researchers matched these women with 793 women who did not have breast cancer. The women were interviewed in person about known or suspected breast cancer risk factors, including family history of breast cancer, alcohol consumption, and cigarette smoking. Additionally, questions were asked that pertained to lifetime occupational history as well as the participants' sleep habits and patterns in the decade prior to diagnosis.

Researchers found an increased risk of breast cancer among participants who reported not sleeping during the period of the night when melatonin levels are typically highest—defined for this study as between 1:00 and 2:00 a.m. Additionally, women who reported working the graveyard shift at least some time in the 10 years leading up to the diagnosis of breast cancer had an increase in risk. That risk further increased as the number of years of graveyard shift work increased and as the hours per week of work during the graveyard shift increased.

In the second study, researchers analyzed data from the Nurses' Health Study—an ongoing study that includes more than 78,000 women. The women involved in this study, which began in 1976, complete questionnaires every two years about health status, medical history, and known or suspected risk factors for cancer and heart disease. In 1988, the women also answered questions about night-shift work.

In the following 10 years, there were 2,441 cases of breast cancer. Using this information, researchers found that the women who worked between 1 and 29 years on a rotating night-shift schedule had an 8% increase in breast cancer risk. If a woman worked for 30 years or more on the night shift, her risk increased by 36%.

Both studies, although different in design, show similar results. In a related editorial, Johnni Hansen, PhD, of the Danish Cancer Society, notes that "evidence is accumulating for an association between exposure to light at night and breast cancer risk."

Dr. Hansen suggests that the link may have something to do with melatonin—a hormone produced by the pineal gland whose production typically peaks during nighttime sleep. Melatonin is involved in circadian rhythms and sleep. When normal melatonin cycles are interrupted in women, the ovaries tend to produce more estrogen. This may contribute to the increase in breast cancer risk.

The combined results may seem alarming, especially in light of the increase in irregular work hours in today's society. But, these are preliminary studies. They do not prove a relationship; what they do show, according to Dr. Hansen, "is an urgent need for further exploration."