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Schichtarbeit und Krebs?
Aktueller Kenntnisstand und offene Fragen

In 2007, the International Agency for Research on Cancer (IARC) classified shift work which involves disruption of circadian rhythms as probably carcinogenic to humans. Since then, among the possible effects of shift work on health the potential association with cancer, especially breast cancer, has gained particular public attention.
Results from two large US prospective cohort studies of a total of about 200,000 nurses are of special interest; among those nurses who worked rotating night shifts for many years more breast cancer cases were observed than expected. Although in these studies many potential confounders were taken into account, other occupational risk factors such as X-ray exposure or handling of chemotherapeutics (which could not be addressed) might have affected the observed associations. Any interpretation of the results of experimental studies in animals with respect to a potential carcinogenic effect of shift work in humans needs to consider that exposure conditions such as constant light exposure or constant darkness are not an adequate correlate of shift work involving work at night. There are theoretical considerations of possible mechanisms, but it has not yet been demonstrated that the mechanisms discussed to date do indeed lead to cancer and are relevant for humans. For example, the question as to whether specific shift schedules cause continuous reductions in melatonin concentrations which then lead to raised oestrogen levels favouring the development of breast cancer – as one of the theories postulates – has not yet been adequately studied.
To date, there is not sufficient evidence for a carcinogenic effect of shift work (especially night work). Large prospective cohort studies involving different occupational groups and precise assessment of shift work exposure and covariates such as sleep quality and lifestyle are required for more insight into the relationship between shift work and cancer. In addition, cell culture experiments and animal studies in which the exposures optimally reflect human shift work may help to explore underlying mechanisms.
Most importantly, prevention requires the identification of those shift schedules with the least adverse effects on health. As a further goal, the non-avoidable disturbing influences of shift work on circadian rhythms should be minimized.

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