Are Women Who Work Night Shifts at a Higher Risk for Developing Breast Cancer?

Night shift work may seem like an odd thing to link to breast cancer. Nevertheless, scientists found that women who work night shifts for many years are more likely to get breast cancer than other women. This includes nurses and flight attendants who work overnight. The International Agency for Research on Cancer, which is a part of the World Health Organization, reported that shift work is a likely risk factor for certain cancers, just as diet can increase or decrease the risks. Researchers found that night shift work links to breast cancer because it can change a person’s sleep-wake cycle. This has a lot to do with artificial light.[1]

What does the research say about the link between night shift work and breast cancer?

Researchers have studied this question in different ways and have come to different conclusions. This can be confusing. One way researchers can help make sense of different conclusions is to combine multiple studies into a larger combination study. Six groups of researchers in the past decade have used these larger studies to ask if night shift work affects breast cancer risk. Five of these studies found that the risk of breast cancer increased by between 5% and 20%.

The number of years a woman has worked night shifts also seems to matter. One team found that a woman’s risk for breast cancer increased 3% for every 5 years that she worked night shifts and 13% for every 500 night shifts worked.[2] Johns Hopkins University researchers found that women who “regularly” worked night shifts increased their risk for developing breast cancer by 20% compared with women who did not.[3] A third group found that women who worked night shifts for fewer than 5 years had a 2% increase in  risk. However, those that worked night shifts for over 20 years had a 9% increase in risk.[4]

One of the larger studies, done in 2016 by a University of Oxford research team,  made the news because they did not find a link between night shift work and breast cancer risk.[5] However, experts on this topic quickly criticized this study for the way it was designed[6]  For example, the Oxford researchers used studies that only followed women for 2 to 4 years. This is much shorter than the previous studies that found a link between breast cancer and shift work. Following women for only 2 to 4 years is not enough time to see if women’s risk of breast cancer risk will change.

Another major problem with the Oxford study had to do with confusing survey responses measuring how often a woman worked night shifts. This was a significant flaw in the study.

What does this increased risk mean?

The average woman has a 1 in 8 chance, or 12.4% chance, of getting breast cancer at some point during her life.[7] In addition, a woman’s risk of developing breast cancer increases as she ages. Working night shifts for a long time increases risk by between 5% and 20% of a woman’s current risk.  So, for a woman working night shifts, her risk would increase to about 13%-14%. This is a small increase in risk for the average woman. However, any increased risk is of concern for women that have other risk factors for breast cancer, such as a family history of breast cancer or mutations of the BRCA1 or BRCA2 genes (often called the “breast cancer genes”).

Why is night shift work linked to breast cancer?

Night shift work can change a person’s regular sleep-wake cycle. Our sleep-wake cycle is a roughly 24-hour rhythm that tells us when we are alert or sleepy.[8] Humans are naturally active during the day and sleepy at night. However, women who work night shifts reverse this pattern. When a woman is working night shifts, she might use external signals, like artificial light or caffeine, to help tell her body to stay awake. The problem is that her body still sends internal signals that it is time for sleep. These different signals disrupt her natural sleep-wake cycle.

Hormones and other bodily activities do not change to match the woman’s work schedule either. Some of these hormones affect tumors, so this can allow tumors to grow.[9]  One example is the melatonin that our bodies make at night to help us sleep.[8] Melatonin helps to prevent tumor growth.[9] A woman who works in artificial light at night makes less melatonin. Another example is glucocorticoids, which our bodies make when we are stressed. People who work night shifts have higher levels of glucocorticoids that help tumors survive.

Why is this important?

Breast cancer is the most common form of cancer in women.[10] We know that 1 in 8 U.S. women will develop invasive breast cancer over the course of her lifetime. In this next year, 255,000 women in the U.S. will be diagnosed with invasive breast cancer. Another 63,410 will be diagnosed with ductal carcinoma in situ, a non-invasive condition when abnormal cells develop in the milk ducts in the breast).[11]  The number of people that work on night shifts full-time is increasing. In 2004, there were 15 million Americans.[12] This is concerning for women who work night shifts over a long period of time because they may be at an increased risk for developing breast cancer.

All articles are reviewed and approved by Dr. Diana Zuckerman and other senior staff.


  1. International Agency for Research on Cancer (IARC). (2007). IARC monographs on the evaluation of carcinogenic risks to humans. Volume 98. Shift-work, painting and fire-fighting. Lyon: International Agency for Research on Cancer.
  2. Wang, F., Yeung, K. L., Chan, W. C., Kwok, C. C., Leung, S. L., Wu, C., Chan, E. Y. Y., Yu, I. T. S., Yang, X. R., & Tse, L. A. (2013). A meta-analysis on dose-response relationship between night shift work and the risk of breast cancer. Annals of Oncology, 24(11), 2724-2732. doi:10.1093/annonc/mdt283.
  3. Kamdar, B. B., Tergas, A. I., Mateen, F. J., Bhayani, N. H., & Oh, J. (2013). Night-shift work and risk of breast cancer: a systematic review and meta-analysis. Breast Cancer Research and Treatment, 138(1), 291-301. doi:10.1007/s10549-013-2433-1.
  4. Lin, X., Chen, W., Wei, F., Ying, M., Wei, W., & Xie, X. (2015). Night-shift work increases morbidity of breast cancer and all-cause mortality: a meta-analysis of 16 prospective cohort studies. Sleep Medicine, 16(11), 1381-1387. doi:10.1016/j.sleep.2015.02.543.
  5. Travis, R. C., Balkwill, A., Fensom, G. K., Appleby, P. N., Reeves, G. K., Wang, X., Roddam, A. W., Gathani, T., Peto, R., Green, J., Key, T. J., & Beral, V. (2016). Night Shift Work and Breast Cancer Incidence: Three Prospective Studies and Meta-analysis of Published Studies. Journal of the National Cancer Institute, 108(12). doi:10.1093/jnci/djw169.
  6. Hazards Magazine special online report. (2016, December). Cancer all-clear for night work based on ‘bad science’, warn scientists. Retrieved from
  7. National Cancer Institute at the National Institute for Health (NIH). (2012). Breast Cancer Risk in American Women. Retrieved from
  8. National Institute of General Medical Sciences at the National Institute of Health (NIH). Circadian Rhythms Fact Sheet. (2012, November). Retrieved from
  9. Ball, L. J., Palesh, O., & Kriegsfeld, L. J. (2016). The Pathophysiologic Role of Disrupted Circadian and Neuroendocrine Rhythms in Breast Carcinogenesis. Endocrine Reviews, 37(5), 450-466.
  10. World Health Organization (WHO). (2017). Breast cancer: prevention and control. Retrieved from
  11. org. (2017, January 10). U.S. Breast Cancer Statistics. Retrieved from
  12. The National Institute for Occupational Safety and Health (NIOSH) at the Centers for Disease Control and Prevention (CDC). (2016, June 21). Work Schedules: Shift Work and Long Hours. Retrieved from