Shift work is increasingly regarded as hazardous to sleep and health. Shift work involves schedules that deviate from the standard 9-5 daytime workweek. Many people who work shifts are on rotating schedules that include both days and nights, and others work consistently late nights, overnights, or very early mornings. These schedules are frequently disruptive to circadian rhythms and make it difficult for people to get sufficient amounts of high-quality sleep. Shift work is associated with elevated risks for a number of serious diseases, including heart disease, diabetes, and several types of cancer.
New research indicates that men who engage in shift work are at higher risk for PSA (prostate-specific antigen), a protein that is produced by the prostate gland. PSA tests are used to screen for prostate cancer and other prostate conditions. Elevated levels of PSA are considered an indicator of possible prostate cancer as well as several other non-cancerous conditions, including prostatitis and enlarged prostate.
Researchers at Harvard University and the University of Connecticut investigated the link between shift work and PSA levels. Researchers used data from the National Health and Nutrition Examination Survey (NHANES), an ongoing national health study conducted by the Centers for Disease Control. Researchers examined data from several NHANES surveys conducted during the years 2005-2010, to create a group of 2017 men who formed the basis for their inquiry. The men were between the ages 40-65, and none had any prior history of cancer. (An exception was made for non-melanoma skin cancer.) All the men had a current PSA test result. The group included a combination of men working regular daytime schedules and men working shifts, both overnight and rotating night and day shifts. Researchers' analysis revealed that men working shifts were significantly more likely to have elevated PSA levels:
- Among the group as a whole, 3 percent of men had PSA of 4.0 ng/mL or higher. "Normal" PSA levels depend on several factors, including age -- but PSA at or above 4.0 has traditionally been considered abnormal, and often leads to further testing for prostate cancer.
- Men who worked shifts were approximately 2.5 times as likely to have a PSA at 4.0 ng/mL or higher, compared to men who were engaged in non-shift work.
It's important to note that this study did not examine the risk of prostate cancer in relation to shift work, only the relationship of shift work to PSA levels. An elevated PSA is not itself a diagnosis of prostate cancer, and high PSA results can be indicative of other, non-cancerous conditions. But an elevated PSA can indicate the presence of prostate cancer. And other research has established links between shift work and prostate cancer, as well as other cancers in both men and women:
- Canadian scientists studied the relationship between shift work and several types of cancers. They found men who worked night shifts had more than 2.5 times the risk for prostate cancer compared to men who had never worked night shifts. Men working night shifts were also at elevated risk for several other types of cancer, including cancers of the pancreas, rectum, colon, bladder, and lungs. Several other cancers showed no increased risk among night shift workers.
- A study in Japan of more than 14,000 workers found higher risk of prostate cancer among men who worked night shifts compared to those who worked days.
- Several studies have explored links between shift work and breast cancer in women. One recent study found that women who worked night shifts for 30 years or more had twice the risk of developing breast cancer. Women who worked less than 30 years on night shifts did not show an increased risk for developing breast cancer.
- Research has also found night shift work linked on an increased risk of ovarian cancer. This study found an increased risk for ovarian cancer among women 50 years and older who worked nights.
What is behind the cancer risks associated with shift work? We don't yet know, but several possible causes are being explored. Several of these focus on the circadian disruptions associated with shift work, including prolonged exposure to artificial light at night, hormonal changes associated with circadian disruptions, and the role of melatonin in tumor growth.
The relationship of circadian dysfunction to cancer risk is a critically important area of research. With millions of Americans working shifts -- and a wider array of jobs requiring non-traditional schedules -- this is an issue that needs rigorous study and attention.
Michael J. Breus, PhD
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