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Title: A short-term increase in cancer risk associated with daytime napping is likely to reflect pre-clinical disease: prospective cohort study

Authors: B.J. Cairns, R.C. Travis, X.-S. Wang, G.K. Reeves, J. Green and V. Beral on behalf of the Million Women Study Collaborators

Reference: British Journal of Cancer 107:527-530.

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Background: Sleep disturbance, a correlate of which is daytime napping, has been hypothesised to be associated with risk of breast and other cancers.

Methods: We estimated relative risks (RR) of breast and other invasive cancers by the reported frequency of daytime napping in a large prospective cohort of middle-aged women in the UK.

Results: During an average of 7.4 years of follow-up, 20 058 breast cancers and 31 856 other cancers were diagnosed. Over the first 4 years of follow-up, daytime napping (sometimes/usually vs rarely/never) was associated with slightly increased risks of breast cancer (RR=1.10, 95% CI 1.061.15) and of other cancers (RR=1.12, 1.081.15), but the RRs decreased significantly with increasing follow-up time (P=0.001 and P=0.01, respectively, for trend). Four or more years after baseline, there was no elevated risk of breast cancer (RR=1.00, 0.961.05), and only marginally greater risk of other cancers (RR=1.04, 1.011.07).

Conclusion: The effect of pre-clinical disease is a likely explanation for the short-term increased risk of breast and other cancers associated with daytime napping.

Keywords. daytime napping; sleep disturbance; breast cancer; reverse causation; pre-clinical cancer

Acknowledgement. We thank all the women who participated in the Million Women Study, collaborators from the NHS Breast Screening Centres, the study steering committee, and the members of the study co-ordinating centre (see Supplementary Appendix). The Million Women Study is supported by the Cancer Research UK, the UK Medical Research Council, the UK National Health Service breast cancer screening programme, and the UK Health and Safety Executive. The funders did not influence the conduct of the study, the preparation of this report, or the decision to publish.

© 2002-2017 Benjamin J. Cairns: e-mail ; ph +44 1865 289673 ;
Cancer Epidemiology Unit, University of Oxford, Richard Doll Building, Roosevelt Drive, Oxford OX3 7LF, U.K.