Shift work that involves circadian disruption and breast cancer: a first application of chronobiological theory and the consequent challenges

Aug 5, 2017Occupational and environmental medicine

Shift work that disrupts the body’s internal clock and its link to breast cancer: applying time-based biological theory and related challenges

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Abstract

Only 30 of 1385 night jobs changed classification based on chronotype.

The overall breast cancer risk associated with shift work remained similar when chronotype was considered (OR 1.17, 95% CI 0.98 to 1.41).
Chronotype, or an individual's preferred bedtime, was used to reclassify exposure to circadian disruption in the study.
No significant differences were observed between the new and old classifications of exposure to night shifts.
Challenges exist in quantifying the total number of chronodisrupted shifts over a lifetime.
The study highlights the complexity of assessing the impact of work schedules on health outcomes.

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OBJECTIVES: In 2007, the International Agency for Research on Cancer classified shift work involving circadian disruption (CD) as probably carcinogenic to humans. Circadian disruption could be conceptualised as the overlap of activity, such as work, with an individual's biological night. The latter can be approximated from a worker's chronotype (or morning/evening preference). Few previous studies have taken chronotype into account when assessing CD caused by shift work. Our objective was to test the hypothesis that women working during their biological night would be at increased risk of breast cancer.

METHODS: We used data from our case-control study of breast cancer to investigate associations between shift work involving CD and breast cancer risks. Previously, we had assumed that everyone working in jobs which involved work for two or more shifts between midnight and 05:00 hours was equally exposed to CD. In the present analyses, we reclassified as unexposed those who had a late chronotype in which their preferred bedtime was 2 hours after the end of their shift.

RESULTS: Only 30 of 1385 night jobs changed classification and the overall finding (OR 1.17, 95% CI 0.98 to 1.41) was not different to the original finding when chronotype was not considered.

CONCLUSIONS: We found virtually no difference between our new and old classifications of exposure. However, we were not able to calculate the total number of chronodisrupted shifts over a lifetime in order to assess dose and nor were we able to determine how many women were exposed to CD when doing shifts which began before midnight. Our first practical application highlights challenges for future chronobiology-based research.

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Shift work that involves circadian disruption and breast cancer: a first application of chronobiological theory and the consequent challenges

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