Researchers at the University of Oslo and the Cancer Registry of Norway (CRN) have found that exposure to night shift work, particularly long-term exposure to rollover shift work, is associated with an increased risk of aggressive prostate cancer in offshore petroleum workers.
Day and night shift rotation involves working seven night shifts, followed by seven day shifts, or vice versa, during the course of each 14-day work period.
With 5,000 cases diagnosed yearly, prostate cancer is the most frequent cancer among men in Norway. Previous studies have shown that people working in the Norwegian offshore petroleum industry have a 20% higher risk of prostate cancer than the rest of the Norwegian population.
These earlier studies used data from the Norwegian Offshore Petroleum Workers (NOPW) cohort, which contains detailed work histories for over 25, 000 male offshore workers employed on the Norwegian continental shelf from 1965–1998. The Cancer Registry of Norway monitored the participants between 1999 and 2019 for potential diagnoses of aggressive prostate cancer.
Research elsewhere provided the Norwegian team with other clues. For example, the International Agency for Research on Cancer, Lyon, France, recently reconfirmed its classification of night shift work as a probable human carcinogen, with limited positive evidence for prostate cancer.
Cancer risk increases when the balance between light and dark is continually disturbed.
Other studies quoted by the researchers in their recent International Journal of Epidemiology paper show inconsistent results, although positive associations have been found for aggressive prostate cancer and longer durations of night shift work exposure.
However, the authors, who collaborated with researchers at the U.S. National Cancer Institute, Bethesda, Md., and the Albert Einstein College of Medicine, New York City, believe this study is the first to investigate the association between rotating shift patterns and the hazard of aggressive prostate cancer.
The study also examined chemical co-exposures to chlorinated degreasing agents as potential endocrine disrupters. These subsequently were found to be not associated with aggressive prostate cancer hazard.
“Of 25,000 male participants in the study, all of whom had worked on oil platforms, 300 were diagnosed with aggressive prostate cancer. We studied offshore petroleum workers who rotated between working night and day shifts and compared them to those who only worked day shifts. We discovered that the workers who worked rotating shifts had an increased risk of getting aggressive prostate cancer," says Leon Alexander Mclaren Berge, a postdoctoral fellow in the university’s Department of Biostatistics and the CRN.
"We found that those who had worked for more than 19 years on a rotating shift schedule of this kind had an 86% higher risk of getting aggressive prostate cancer compared to those who only worked day shifts. But this estimate must be cautiously interpreted, since there were relatively few cancer cases in this analysis," he adds.
However, the researchers believe the increased risk may be due to regular disruption of the sleep/wake cycle over long periods. As Berge notes, animal experiments have shown cancer risk increases when the balance between light and dark is continually disturbed.
Hormones could be a risk factor, too. Prostate cancer is believed to be particularly sensitive to hormones and people who work nights are known to have low levels of melatonin. According to the U.S. National Institutes of Health (NIH), melatonin helps with the timing of circadian rhythms — the 24-hour internal clock — and with sleep. Exposure to light at night can block melatonin production, cautions the NIH.
While many other studies on night shift work have been carried out, Berge notes that researchers have not found a connection between prostate cancer and night shift work in all these studies. It may depend on the type of work in question and on the times and lengths of the night shifts, he emphasizes.
"A general recommendation could be for workers to do fewer and shorter night shifts and to space their periods of night work further apart. This would reduce how frequently the sleep and wake cycle is disrupted and would give workers more time to stably readjust to a new rhythm," he concludes.
Nevertheless, the authors acknowledge that a great deal remains unknown about the factors that increase the risk of developing this form of cancer. Researchers believe old age and the occurrence of previous cases of prostate cancer in multiple family members can be contributory factors. Excess weight, smoking, and exposure to certain chemicals may play a role, too.
