sleep duration, other sleep vari es and breast cancer risk factors were assesscd by self administered questionnaires in 1975 and in 1981 ureast cancer incidence data for years 1976-96 was obtained from the nish Cancer Registry, Hazard ratios (HRs), and 95% Cls were obtained from Co roportional hazards models adjusting for potentia confounders. Analysis restricted to 7396 women (146 cases) whose sleep duration in 1975 and 198 the same duration group (stable sleepers) yielded HRs of 1.10 (CI: o.59-2.05) for s6h, 1.0 for T 8 h and 0.28 (CH o.09-0.88) for >9 h, with decreasing trend (P 0.03) The use of self-report for sleep duration may be of only limited value in estimating the association of actual sleep duration and breast cancer extension of whether sleep duration accurately eflects hours of exposure to dark at night. Self report of sleep duration (subjective measure) has been compared with sleep as assessed by actigraphy measure) in a large sample of adults and (objective found to be correlated generally overestimate the objective measure this bias also varied several demographic variables. A detailed by objective analysis of reported sleep characteristics and measures of sleep and circadian phase in blind per found that was well with duration but with number of nocturnal awaken ifigs or haps a Tn this study, there was good agree ment between actigraphy and self-report on circadian phase as measured by urinary 6-sulphatoxymelatonin Since this Finnish study appeared, three more stu dies have been published. Pinheiro et on sleep duration and breast cancer risk in the Nurse Health Study 1, and found no overall association. mong women reporting the same sleep duration on uestionnaires from 1986 and 2000, there