Research indicates night shift workers are at high risk for cancer and heart disease

Many employees are chronically sleep-deprived

If an employee complained about fatigue, insomnia, or difficulty concentrating, you would probably suspect their health problem was due to working the night shift. But what if that worker was diagnosed with cancer?

New evidence of increased cancer rates in night shift employees resulted in overnight shift work being added as a "probable carcinogen" by the International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organization.1 According to 2001 data from the Bureau of Labor Statistics, almost 15 million Americans work evening, night, rotating, or other irregular schedules.

"We know that there are very definite problems that have long been associated with shift work, including hypertension, obesity, and heart disease," says Michael Smolensky, PhD, professor of environmental physiology at the University of Texas School of Public Health in Houston.2,3 "These types of complaints haven't usually been linked to night shift work, but this is coming to the attention of a lot of people right now."

Previous research found links between night shift work and breast and prostate cancer.4,5 Scientists suspect that overnight work is dangerous because it disrupts the circadian rhythm, disturbs the body's biological clock, and may result in lower melatonin levels, which can increase cancer risks, says Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, NY. "The employee's biological clock is being negatively affected," Rea says. "Each cell in the body has a time to perform a particular function, and if you upset that, it is not a healthy thing."

This research is the first strong evidence for a cancer risk for night shift workers, notes Richard Stevens, a cancer epidemiologist and professor at the University of Connecticut Health Center in Farmington. "More research must be done to further specify the magnitude of that risk and what can be done to lower it," Stevens says.

Despite the unanswered questions, occupational health nurses should, at a minimum, keep night shift workers updated on the new research, says Jere Zimmerman, director of environmental health and safety for Coors in Golden, CO. Occupational health managers at Coors were sent a memo on the study and asked to share the information with employees, says Zimmerman. "We feel employees have a right to be informed. Therefore, when this study was published, we provided our shift workers with the study results," says Zimmerman. "We feel each individual should make up their own minds. We have encouraged concerned employees to seek the advice of their medical doctor if they have concerns."

At Cincinnati-based Proctor & Gamble, occupational health professionals are evaluating the IARC's report, says spokesman Doug Shelton. "We want to ensure we have the right approach to protecting the health and well-being of our employees," Shelton says. "We will communicate our findings to employees as soon as we have completed our analysis."

Surprisingly, many occupational health professionals are unaware of the health risks of night shift work, says Smolensky. "I get calls from occupational health doctors who wonder why workers are complaining about inability to sleep or depression," he says. "They want to know if they're for real or are they pulling our leg to get off on workers' comp. There is not a lot of familiarity with this. Occupational health professionals don't really understand the health concerns and what to look out for with these workers."

When addressing health risks of night shift workers, consider the following:

  • Workers who switch between day and night work may be at even higher risk. "If people work the night shift irregularly for a few days on and off, that appears to be particularly detrimental," says Rea. "It appears that this shifting back and forth confuses the body. If you can stay anchored in a 'day shift world' but spend a few nights working, that appears less problematic." Most night shift workers are not willing to convert to a completely "night shift world," says Rea. In this case, he says, it's probably better to reduce the number of nights on the night shift, such as doing two nights, then taking three to five days off. "Your body usually wants to stay on a day shift. So as long as you have an anchor to come back to, it's better if you do rotating shift work," says Rea. "The worst thing you can do is prolonged night shift with prolonged day shift; that is a very irregular pattern to avoid."
  • Long-term melatonin supplements aren't recommended. Exposure to light while working at night inhibits melatonin production, which is not good because it has oncologic preventive properties, says Smolensky. So should night shift workers take melatonin supplements? "It's not a good idea to take melatonin on a regular basis because it is a hormone and a pill spikes melatonin far higher than is normal," says Stevens. "It is better to live a 'melatonin-friendly' lifestyle by getting eight or nine hours of dark, exercise in the morning, and a good diet." Likewise, there is no evidence that exposure to bright lights during a night shift is helpful, says Rea. "That might not be a good idea, unless, again, you are willing to completely change your life over so you are awake at night and asleep during the day. And most people are unwilling to do that," he says.
  • Workers are at higher risk for accidents. Night shift work is linked to an increased risk of automobile and workplace accidents, says Smolensky.6, 7 In addition, research on workers' comp claims found that there were increased filings and greater cost per accident for the night shift, he says.8 "Some of these accidents are the result of disruption in circadian rhythms, which impact hand-eye coordination and cause fatigue," says Smolensky.
  • Sleep deprivation could be a factor in increased cancer risk. If employees are not getting enough sleep, their immune system is vulnerable to attack, and they are less able to fight off potentially cancerous cells, says Rea. Many night shift workers only get four to five hours sleep a night, so they are chronically sleep-deprived, says Smolensky. "A lot of people who work the night shift eventually have problems sleeping. Often, they resort to using alcohol or over-the-counter medications, which causes more problems." When night shift workers get off work, sleeping in a darkened room can help stave off insomnia, says Stevens. "It is best to sleep in a very dark place and keep it dark for eight or nine hours if possible," he says.


1. Straif K, Baan R, Grosse Y, et al. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Onc 2007; 8:1,065-1,066.

2. Kawachi I, Colditz GA, Stampfer MJ, et al. Prospective study of shift work and risk of coronary heart disease in women. Circulation 1995; 92:3,178-3,182.

3. Pickering TG. Could hypertension be a consequence of the 24/7 society? The effects of sleep deprivation and shift work. J Clin Hypertension 2006; 8:819–822.

4. Schernhammer ES, et al. Rotating night shifts and risk of breast cancer in women participating in the Nurses' Health Study. J Natl Cancer Inst 2001; 93:1,563-1,568.

5. Kubo T, Ozasa K, Mikami K, et al. Prospective cohort study of the risk of prostate cancer among rotating-shift workers: Findings from the Japan collaborative cohort study. Amer J Epidem 2006; 164:549-555.

6. Frank AL. Injuries related to shift work. Amer J Prev Med 2000; 18:33-36.

7. Yueng-Hsiang H, Chiuan J C, DeArmond S, et al. Role of safety climate and shift work on perceived injury risk: A multi-level analysis. Accid Anal Prev 2007; 39:1,088-1,096.

8. Horwitz IB, McCall BP. The effects of shift work on the occupational safety and health of hospital employees: A risk and severity analysis using Oregon workers' compensation data. Occ Med 2004; 54:556-563.


For more information on health risks for night shift workers, contact:

  • The National Institute for Occupational Safety & Health has a variety of publications and other resources for shift workers on its web site ( At the top, click on "All NIOSH Topics" and then "Work Schedules: Shift Work and Long Work Hours."
  • The National Sleep Foundation offers recommendations for employees who work the night shift on its web site ( Under the "Resources" box on the left side of the page, click on "Topics: A to Zzzzs" and scroll down and click on "Shift Work" and then "Strategies for Shift Workers."