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

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 (RR1.10, 95% CI 1.06-1.15) and of other cancers (RR1.12, 1.08-1.15), but the RRs decreased significantly with increasing follow-up time (P<0.001 and P