Objectives There is mounting evidence that night shiftwork may increase the incidence of female breast cancer. The influence of night shift work on survival of breast cancer has, however, not been reported. The aim of the present study is to elucidate breast cancer survival in different types of former non-day shift-workers compared to day-workers.
Method In total 1157 women (23% nurses), aged less than 75 years, diagnosed with breast cancer (2000–2004) participated in two independent nationwide case-control studies on night shiftwork. Information on the entire work life, including night shiftwork and potential risk factors for breast cancer (e.g. reproduction, BMI, alcohol, HRT, heredity and diurnal preference) was obtained by telephone interviews. All study subjects were followed up for death in the National Cause of Death Register until end of 2011.Cox proportionate hazard models and Kaplan-Meier survival plots were used to perform time-to-event analyses.
Results In total 127 breast cancer cases (11.0%) had died from this disease at end of follow-up (median follow-up 12.6 years). There was a significant tendency of decreasing survival of breast cancer among both fixed and rotating nightshifts workers compared to daytime shiftworkers and by increasing years of prior non-day time work (p = 0.04). Evening workers had about same survival as day workers. The results were only slightly affected by confounders.
Conclusions These data suggest that night shift work prior to breast cancer seems to decrease survival. The association was not strongly modified by lifestyle factors.