It’s Not Nice to Ignore Mother Nature: Night Workers May Pay a Price

Abstract & commentary

Synopsis: Working at night appears to increase the risk of breast cancer for women by about 50%.

Source: Hansen J. Epidemiology. 2001;12:74-77.

This was a retrospective analysis of 7035 danish female breast cancer patients compared to age-matched controls without breast cancer. Cases were identified from the Danish Cancer Registry. Of the 7565 women with breast cancer born between 1935 and 1959, 530 (7%) were excluded from this study because they had never been part of the workforce, leaving 7035. Controls were drawn from the files of the Central Population Registry. Data used to classify which workers were predominant night workers came from an earlier interview-based survey. Women were considered to work predominantly at night if they had been employed for at least half a year in one or more of the trades in which at least 60% of the female responders had nighttime schedules. Hansen controlled for reproductive history, alcohol consumption, and socioeconomic status. For all night workers combined, the odds ratio for breast cancer was 1.5 (CI 1.3-1.7). There was a trend for increasing risk of breast cancer with increasing duration of work at night. Those women with more than 6 years of employment in trades with predominantly night work had a 1.7-fold increased risk of breast cancer.

Comment by Barbara A. Phillips, MD, MSPH

In the United States, as in Denmark, about 20% of the workforce does some kind of night work.1 While shiftworkers have long been known to have reduced quality and quantity of sleep compared with day workers,2 we are just beginning to gather data on the health consequences of night work. The relationship of shiftwork to gastrointestinal and cardiovascular disease is fairly well established, although the mechanisms remain unclear.3-6

The current study opens a whole new Pandora’s box of potential occupational illness. It builds on earlier work indicating that female nurses,7,8 flight attendants,9,10 and radio and telegraph workers11 who worked nights have increased rates of breast cancer. What is new and important about Hansen’s study is that it was designed to test the relationship of night work to breast cancer, and that it controlled for the well-known confounding risk factors of parity, alcohol use, and socioeconomic status.

Further, Hansen puts forth a plausible hypothesis supported by data: increased ocular exposure to visible light decreases blood melatonin levels,12 and melatonin suppresses human breast cancer cell growth.13,14 Further, estrogen-receptor-positive breast cancer patients appear to have low serum melatonin concentrations. Of further relevance is the fact that totally blind women, who do not undergo light-induced inhibition of melatonin, have lower breast cancer rates than do other women.15

This study is important because it helps us to better understand the pathogenesis of the No. 2 cancer killer of women, and it provides evidence that melatonin is actually a sex hormone. Maybe it should also remind us that it’s unwise to disregard the cycles of the earth’s rotation—we are meant to sleep at night!


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