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Night work and breast cancer risk in a cohort of female healthcare employees in Stockholm, Sweden
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  • Published on:
    • Thomas Erren, Physician epidemiologist Institute & Policlinic for Occupational Medicine, Environmental Medicine and Prevention Research, University Hospital of Cologne
    • Other Contributors:
      • Peter Morfeld, Statistician epidemiologist

    With interest we read the article by Gustavsson and colleagues [1] on the breast cancer risk in a cohort with night work. The authors started from two facts: First, “night shift work” [2] was classified as “probably carcinogenic to humans” (Group 2A) by the International Agency for Research on Cancer [IARC]; second, the evidence in humans was considered limited because of variable results and potential bias. Since prior studies had problems regarding exposure assessment, Gustavsson et al. emphasized their very detailed registry-based data on night work. Yet, as key result the authors noted that “conclusions are limited due to a short period of follow-up and lack of information of night work before 2008”. Thus, this study perpetuates limited epidemiological evidence for the carcinogenicity of night work. Although the limited data on shift work is a drawback of this study, it is not the only limitation. We would like to discuss a conceptual problem that may have contributed to the limited conclusions and that the authors did not address.
    The IARC monograph mentions chronotype and sleep 73 and 199 times, respectively [2]. Chronotype tells us when persons prefer sleep or work and activity. Potentially harmful circadian disruption (CD) [3] can occur at any time over 24 hours when activities or sleep are misaligned with the chronotype-associated biological nights [3 4] or biological days. This leads to occupational and non-occupational CD [5]. Possible effects of not c...

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    Conflict of Interest:
    None declared.