Scientific Symposium - The Health Effects of Shift Work - Toronto, April 12, 2010

Shift work and breast cancer: the need for mechanisms

Richard G Stevens


Mounting evidence supports the hypothesis that non-day shift work increases risk of breast cancer in women. The IARC has classified 'shift work' as a probable human carcinogen (2A) based on the published data. In addition, a number of other large studies are ongoing or recently finished and will be reported soon. If these also support the hypothesis, and a consensus emerges that in fact, shift does increase risk (IARC class 1 carcinogen), then it will become imperative to understand the mechanism(s) for the effect. Shift work is a necessity for many people, and will only rise in prevalence as the world increasingly operates on a 24-hour global economy. There are many possible mechanisms, two prominent of which are misalignment of melatonin rhythms, and disruption of circadian gene function. These are not mutually exclusive, and there is evidence for each from both human and laboratory research.

Presenter Biography

Richard Stevens received a B.S. in Genetics from the University of California, Berkeley, and a Ph.D. in Epidemiology from the University of Washington in Seattle. He has been working for a long time trying to help figure out why people get cancer. One of his major interests has been in the possible role of iron overload. Largely on the basis of his work, published in the Journal of National Cancer Institute and the New England Journal of Medicine, the Swedish food industry decided to cease iron fortification of flour in the early 1990s. A perplexing challenge which Stevens began to engage in the late 1970s is the confounding mystery of why breast cancer risk rises so dramatically as societies industrialize. He proposed in 1987 a radical new theory that use of electric lighting, resulting in lighted nights, might produce 'circadian disruption' causing changes in the hormones relevant to breast cancer risk, and thereby play an important role in breast cancer causation worldwide. Accumulating evidence has generally supported the theory. Women who have an occupation requiring work in the evening or at night are at higher risk; blind women have been reported to be at reduced risk; and a new study from Finland has found women who sleep longer than average have much lower risk of breast cancer. Dr. Stevens' theory has received wide scientific and public attention. For example, his work has been featured in SCIENCE NEWS (October 17, 1998 and January 7, 2006) and on the cover of the scientific journal CANCER RESEARCH (July 15, 1996) as well as cited in the March 24, 2008 issue of US News & World Report ('Turning Out the Lights' by Ben Harder), the August 20, 2007 issue of the New Yorker ('The Dark Side' by David Owen), and the August, 2008 issue of O Magazine ('Bright Lights, Big Risk for Cancer' by Catherine Guthrie).



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 Occupational Cancer Research CentreInstitute for Work & Health | Research Excellence Advancing Employee Health