CA Cancer J Clin 2002; 52:4
doi: 10.3322/canjclin.52.1.4
© 2002 American Cancer Society
WORKING THE NIGHT SHIFT MAY INCREASE BREAST CANCER RISK
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Light at night may increase risk of breast cancer by suppressing melatonin production.
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Results of two recent studies suggest that women who work the night shift may be at an increased risk of developing breast cancer, possibly because prolonged exposure to light at night interferes with production of melatonin, a hormone produced in response to darkness. The reports were published in the Journal of the National Cancer Institute (2001;93:1557-1568). Although one was a case-control study and the other a prospective cohort study, the two reached the same conclusions.
While both studies found that light at night may increase the risk of breast cancer, both lead authors say that the results are still not strong enough to make broad workplace recommendations. "Based on the results of this study, it is too early to say that [working at night] ‘causes' breast cancer, or to know what aspects of working nights is responsible for the increased risk we observe," says Scott Davis, PhD, of the University of Washington in Seattle and the Fred Hutchinson Cancer Research Center. "We really don’t know what aspects of either working nights or being up at night are responsible for the increase in risk we see," Davis says. According to his article, one theory has been that light at night suppresses the normal nocturnal production of melatonin by the pineal gland, which in turn might increase estrogen levels in the body.
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The Case-control Study
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Davis and his coauthors interviewed 813 women who had been diagnosed with breast cancer between 1992 and 1995, and a control group of 793 women who did not have breast cancer. Among women who did not sleep during the hours when melatonin production is usually highest, there was a 14% increase in risk for each day of nonpeak sleep per week. Working the "graveyard shift," defined as starting work after 7:00 PM and leaving before 9:00 AM for at least one day per week over a period of at least three years, increased risk by 60 percent.
"On the other hand, we do know from a large body of literature that there are things people who work [night] shifts can do to help minimize the physical effects," Davis says. "One of these is to sleep in a dark and quiet environment when they do sleep during the day. This seems to enhance the quality of their sleep, and for some, to minimize the adverse physical effects of working at night. However, I must stress, there is no evidence that such practice will influence [reduce] the risk of developing breast cancer."
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The Prospective Cohort Study
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In the second study, Eva S. Schernhammer, MD, MPH, and her colleagues at Brigham and Women's Hospital and Harvard University, used data from the Nurses' Health Study, which surveys a cohort of more than 100,000 nurses on a range of health issues. For this study, Schernhammer and her coauthors followed up on 78,562 of the nurses who answered a question on night work in the 1988 survey.
"Our findings would apply to every woman who works at night, as long as she is exposed to light," Schernhammer says. "How light exposure at night may affect men with regard to a potential cancer risk is unknown at this time, but will be addressed in future studies. Overall, the increased breast cancer risk we found in our study was restricted to a small group of women: those who worked 30 or more years on rotating night shifts—only 1.8 percent of all women in our study population."
"The attributable risk, or public health impact of night work, given confirmation of our results, will therefore be small," she says. "However, because small changes in shift work patterns or light exposure potentially could reduce such risk, it is important to further explore the relation between light at night and breast cancer risk."
Schernhammer says, "Night work is becoming increasingly common. Unfortunately, humans are physiologically unsuited to such irregular schedules. Recent studies suggest that light affects the pineal gland, thus suppressing melatonin production. To sleep in a dim room should then be sufficient protection against light exposure. However, it is premature to draw such conclusions." Schernhammer advises women who work a night shift to see their primary care physicians to discuss their individual risk not just with regard to their sleep habits, but also their weight, family history, alcohol consumption, use of post-menopausal hormones, and other issues.
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Preclinical Studies Suggest Melatonin Role in Tumor Growth
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"These findings are very interesting and deserve additional attention by the research community," says Eugenia E. Calle, PhD, Director of Analytic Epidemiology at the American Cancer Society. "As the authors of both studies acknowledge, the observed association between night work and breast cancer is not large, even among women with many years of night work. Still, cellular studies and studies in animals support a role of melatonin in reducing tumor growth, and we need to know more about what influences circulating melatonin levels and the possible association between these levels and cancer occurrence."
"And given the uncertainties as to the role that melatonin plays in breast cancer, it would be very premature to advise women seeking to decrease their breast cancer risk to take dietary supplements of melatonin."
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The Case-control Study
The Prospective Cohort Study
