NIOSH Training for Nurses on Shift Work and Long Work Hours

Diseases and Shift Work (Continued)

Cancer

In 2007, the International Agency for Research on Cancer of the World Health Organization (WHO) announced that sufficient scientific evidence is available from animal and human studies to label shift work with circadian disruption a “probable” carcinogen.⁶³

Breast cancer has been the most frequently studied type of cancer. According to a 2012 report by Bonde et al.,⁶⁴ statistically significant increases in risk for breast cancer were seen for working night shift for 20 years or more, but it is not clear from existing studies whether or not there is risk for shorter durations. They also suggest that “women with previous or current breast cancer be advised not to work night shifts because of strong experimental evidence demonstrating accelerated tumor growth by suppression of melatonin secretion.” Melatonin secretion is suppressed by light when working at night.

Fewer studies have examined whether there is a link between shift work and other types of cancer. Three of five studies found an increased risk for prostate cancer.^65-69

Costa et al.⁷⁰ discuss mechanisms that could possibly lead to cancer:

• exposure to light at night, which reduces melatonin levels (melatonin combats cancer development and growth)
• repeated shifting of the times of sleep and activity
• depression of immune surveillance for cancer cells.