['Sleep disturbance, a correlate of which is daytime napping, has been hypothesised to be associated with risk of breast and other cancers.', '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.', '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 1.06-1.15) and of other cancers (RR=1.12, 1.08-1.15), 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, 0.96-1.05), and only marginally greater risk of other cancers (RR=1.04, 1.01-1.07).'] Is a short-term increase in cancer risk associated with daytime napping likely to reflect pre-clinical disease : prospective cohort study?