Sleep is critical to our overall health and well-being. Prolonged sleep disruptions, such as those that come with working non-daytime hours, prevent the brain from performing important restorative tasks. A number of studies also suggest long-term shift work is associated with a higher risk of certain types of cancer, especially breast cancer.

If the link is confirmed, night shift work could become a significant public health concern: An estimated 15 to 20 percent of the working population in industrialized countries are employed in some type of permanent night and rotating shift work.

How Shift Work Affects Sleep

Researchers suspect shift work disrupts melatonin, a hormone produced at night during sleep. Melatonin helps control our daily (circadian) rhythms, which regulate sleep-wake cycles and other important functions. Exposure to light at night may suppress melatonin production, which may in turn cause the ovaries to release more of the female hormone estrogen. Prolonged exposure to female hormones a risk factor for breast cancer.

Several years ago, the International Agency for Research on Cancer (IARC) concluded that shift work that disrupts circadian rhythms is probably carcinogenic (cancer-causing) to humans. But it's important to note that the melatonin connection is just one theory; there may be genetic and lifestyle factors at play. For example, studies also show a higher rate of smoking among shift workers—and smoking, as we know, is a significant risk factor for cancer.

At this point, there is limited evidence for a causal relationship between night shift work and breast cancer, and insufficient evidence to suggest that night shift work may lead to prostate, colorectal, or overall cancer.

What Shift Workers Can Do to Minimize Cancer Risk

Additional research is needed to better understand how sleep-wake cycles are controlled and, more importantly for shift workers, what can be done to help them maximize sleep quality and duration, and minimize sleep disruptions.

In the meantime, experts recommend shorter shift lengths and adequate rest breaks. Limiting the total amount of night shift work performed may also help reduce risk: A recent Canadian study found increased breast cancer risk among women employed in night shift work for 30 or more years; there appeared to be no similar risk among those who worked night shifts for shorter periods.


National Cancer Institute. "Sleep Disorders." December 12, 2013.

Scott Davis, Dana K. Mirick and Richard G. Stevens. "Night Shift Work, Light at Night, and Risk of Breast Cancer," Journal of the National Cancer Institute 93(20) (2001): 1557-1562, doi: 10.1093/jnci/93.20.1557.

Lin Fritschi. "Shift Work and Cancer: Short and long term effects provide compelling reasons to act now," Medscape Medical News (2009), accessed February 14, 2014.

Henrik A. Kolstad. "Nightshift work and risk of breast cancer and other cancers.a  critical review of the epidemiological evidence," The Scientific Committee of the Danish Society of Occupational and Environmental Medicine, September 2007, accessed February 14, 2014.

Richard Stevens. "Shift Work - Working against our Internal Clocks," CPH News and Views #11 (2009), accessed February 14, 2014.

Anne Grundy, Harriet Richardson, Igor Burstyn, Caroline Lohrisch, Sandip K SenGupta, Agnes S Lai, Derrick Lee, John J Spinelli, Kristan J Aronson. "Increased Risk of Breast Cancer Associated With Long-term Shift Work in Canada," Occupational and Environmental Medicine 70(12) (2013): 831-838, doi:10.1136/oemed-2013-101482.