Content area
Full Text
What this paper adds The International Agency for Research in Cancer has classified 'shift work that involves circadian disruption' as probably carcinogenic. Circadian disruption may be assessed by taking into account the chronotype (morning/evening type) of the person doing the shifts.
We aimed to reclassify our exposure data from a case-control study of breast cancer by incorporating circadian disruption into the definition of exposure to shift work.
Only a few jobs changed category and we were limited by the data we collected. Future studies should collect data on all shift times not just night shifts and also obtain the number of shifts per year.
Introduction
In 2007, the International Agency for Research on Cancer (IARC) concluded that 'shift work that involves circadian disruption (CD)' is probably carcinogenic to humans based on 'sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)' and 'limited evidence' of increased risks of breast cancer in shift workers in epidemiological studies. 1 The working group did not define the term CD.
The body clock governs circadian rhythms with daily cycles of sleeping, wakefulness, eating, etc. Humans vary in the timing of their genetically determined body clock (chronotype); some people prefer to be awake in the mornings, others prefer the evening. 2 3 This implies that not all workers with the same shifts will experience CD to the same extent. 4 For example, a shift which ends at 02:00 will result in CD for an early chronotype but not for a late chronotype.
Studies examining shift work and breast cancer have had inconsistent results, 5 and one potential reason for this inconsistency may be that the shift-work assessment does not always consider chronotype. To do this, one can estimate each person's biological night and then determine whether the shift they do overlaps with this time window. We would expect that people who work during their biological night would experience CD. 6 In this analysis, we have refined our measure of shift work used in our previously reported case-control study of breast cancer 7 to include CD.
Methods
The methods for the Breast Cancer Environment and Employment Study (BCEES) have been described previously 7 and are summarised here. Approval for the study was obtained from...