Cancer Risk on the Night Shift
Experts say it’s too early to panic
Monday October 6, 2008
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Details at a Glance
• Vitamin D deficiency because of a lack of sun exposure may lead to a higher risk of cancer in night-shift workers.
• Sleep deprivation may negatively impact the immune system and the body's ability to mitigate the production of cancer cells.
• Scientists question whether the timing of light exposure is more important than the duration.
• It is questioned whether working the night shift or other attributes such as body mass index and lack of exercise increase the risk of breast cancer.
• It is most important to address controllable factors proven to increase cancer risk, such as maintaining a lean body weight, physical activity, eating well, quitting smoking, getting regular physical exams and mammograms, and minimizing alcohol consumption.
This conclusion was based on published evidence that includes data collected from the Nurses’ Health Study and the Nurses’ Health Study II. A summary of the evidence was published in the December 2007 issue of The Lancet Oncology. According to the article, 15% to 20% of people working in Europe and the U.S. have jobs that involve night work, and this type of work is prevalent (above 30%) in industries that include healthcare, industrial manufacturing, and mining, to name a few.
“Among the many different patterns of shift work, those including night work are the most disruptive for the circadian clock,” according to the article, and this disruption may be carcinogenic to humans.
Despite this news, nurses and other personnel who staff the graveyard shift should not become alarmed, says Sue Hankinson, RN, ScD, principal investigator of the Nurses’ Health Study and associate professor of medicine at Harvard Medical School in Boston.
Other researchers concur.
“It’s too early to panic,” says Eva Schernhammer, MD, DrPH, coinvestigator for the NHS. “The risks that we see are modest compared to other risk factors for cancer, such as smoking.”
The Hormone of Darkness
In October 2007, a WHO working group met to review the published literature on the topic of shift work, circadian rhythm, and the risk of cancer. To date, most studies in humans have focused on breast cancer in nurses and flight attendants and have found that long-term night workers have a higher risk of breast cancer risk than women who do not work at night.
“It’s largely because of the breast cancer evidence that the International Agency for Research on Cancer committee changed the classification of night work to a probable carcinogen,” says Hankinson.
Researchers also are examining the possible association of shift work to colorectal cancer and endometrial cancer risk.
“We came to the conclusion that while human data are still limited, animal data are fairly consistent and support a mechanistic link between light exposure at night and eventual risk for developing various forms of cancer,” says Schernhammer.
It is believed that artificial light exposure at night does its damage by initiating the suppression of melatonin, also known as the “hormone of darkness.” Melatonin is produced in the brain’s pineal gland during periods of darkness and is believed to inhibit cancer growth.
“Melatonin is produced in a very distinct circadian fashion,” says Schernhammer. “There is almost no production throughout the day in people who have regular circadian rhythms. Most production occurs at night when they are sleeping.”
Other mechanisms that are theorized to lead to a higher risk of cancer in night-shift workers include vitamin D deficiency because of a lack of sun exposure. Sleep deprivation also may negatively impact the immune system and the body’s ability to mitigate the production of cancer cells. Plus, disruption of the circadian rhythm may disregulate genes involved in tumor development.
Unanswered Questions
Researchers have a long way to go to answer questions raised by current evidence. Although they suspect that the longer a person works the night shift the higher the risk for cancer, it is yet to be proved. Scientists also are questioning whether the timing of light exposure is more important than the duration.
“It is possible that there is a certain time in a woman’s life that she is more vulnerable to developing breast cancer from that sort of exposure,” says Schernhammer.
Based on biological principals and current data, researchers hypothesize that working rotating shifts may be more detrimental than maintaining a consistent night-shift schedule.
“We think people who work a stable night shift may be more able to adapt, and ultimately that would mean their melatonin levels are probably more normal than in someone who switches between day and night shifts,” says Schernhammer.
Another question is whether it is the night shift itself or other attributes of women who tend to work the night shift that increases the risk of breast cancer. NHS researchers have been able to account for higher family risk, but not some other variables.
“It’s very difficult to sort out other factors that play a role, such as body mass index and lack of exercise,” says Hankinson.
Researchers also are studying the possibility that some individuals are genetically better able to cope with nighttime exposure to light. It is hoped that someday subgroups that are more vulnerable to the risk will be identified so screening tools can be developed.
Strategies to Minimize Risk
It is too early in the research process for experts to make recommendations to minimize the impact of shift work on the risk of cancer. There are, however, a range of possibilities that are not yet proved. One benign, common-sense option is to use black-out curtains when sleeping during the day.
“Melatonin secretion is very sensitive to the amount of light, so sleeping in a darkened room is a good idea,” says Hankinson.
Other possibilities include working a consistent shift and limiting the number of years worked on the night shift. The use of oral melatonin is not recommended because of a lack of current research about potential long-term side effects. Both Hankinson and Schernhammer agree that, at this juncture, it is most important to address the controllable factors that are proved to increase cancer risk.
“This is good reason to pay more attention to the things that are influential in cancer risk, such as maintaining a lean body weight, [engaging in] physical activity, eating well, quitting smoking, getting regular physical exams and mammograms, and minimizing alcohol consumption,” says Hankinson.
Because of the increasing consistency of the evidence, much of it from the NHS, more studies are expected to be funded soon by the National Institutes of Health.
“The nursing profession is clearly very affected by this type of research, but it is also actively contributing to it — so stay tuned,” says Hankinson.
Catherine Spader, RN, is a contributing writer for Nursing Spectrum/NurseWeek. To comment, e-mail editorNTL@gannetthg.com.
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