Shiftwork/Rotating Schedules Found to Increase Breast Cancer Risk




Journal of the National Cancer Institute

Related Monographs

Consumer Data: Breast Cancer
Professional Data: Breast Cancer


Two studies published in the October issue of the Journal of the National Cancer Institute give rise to serious concern for women working night shifts on an extended basis. The two unrelated studies imply that there is a clear need for researchers to determine if there is a relationship between melatonin and cancer in females. Presently that link is not known.
The study coming from the Fred Hutchinson Cancer Research Center in Seattle Washington along with UW’s School of Public Health, examined the sleep patterns and habits of over 1,600 women between the ages of 20 and 74. Results of this study showed that those women who were not asleep during the fragile hour or two when melatonin levels are highest, faced a breast cancer risk much higher than normal. This risk increased with the number of nights per week that the patient was not asleep during this time period, however the risk did not seem to be affected if sleep during this time was interrupted by turning on a light. Those women who slept with the brightest amount of light were also at a much higher risk. Women who worked the “graveyard” shift were at the highest risk and that risk increased with age and number of years with fluctuating schedules.1
In a completely unrelated study coming from Brigham and Women’s Hospital and Harvard Medical School in Boston, more than 75,000 women participating in the Nurses’ Health Study were followed for 10 years. This study supports the melatonin/breast cancer link and suggests that, again, the risk factor for women who work on rotating night shift was higher than women whose work involved daytime hours. The risk factor increased if women worked as few as three nights per month and the trend in the increase was considered statistically significant.2
There are many unanswered questions regarding the exact role that melatonin plays in breast cancer specifically, but the results of these two studies should be taken seriously by the medical community as well as by women whose work involves rotating schedules or graveyard shiftwork on any level.


1. Davis S, et al. Night Shift Work, Light at Night, and Risk of Breast Cancer. Journal of the National Cancer Institute, Oct 2001.
2. Schernhammer E, et al. Rotating Night Shifts and the Risk of Breast Cancer in Women Participating in the Nurses’ Health Study. Journal of the National Cancer Institute, Oct. 2001.