Recent News about Night Shift Work and Cancer: What Does it Mean for Workers?

The National Toxicology Program (NTP) recently released a report about how persistent night shift work is related to cancer risk (1). This report follows a similar evaluation released in July, 2019 by the International Agency for Research on Cancer (IARC) (2), an update of their 2007 report (3). Both agencies reviewed existing studies of night shift work and cancer and both convened experts in the fields of human, animal, and basic science research. In the recent NTP report, it was concluded that there is “high confidence” that persistent night shift work that results in circadian disruption can cause human cancer, and IARC concluded that night shift work is “probably carcinogenic to humans.” These conclusions are based on evidence from human studies of breast and prostate cancer, studies of laboratory animals, and research into the mechanisms of how cancer develops. Many workers might wonder how night shift work could be related to cancer, and what workers can do to stay healthy.

Biologic clocks and circadian disruption

Many of our biologic systems – including our sleep and wake cycle, our appetites and digestive patterns, our body temperatures, and even our moods – follow daily patterns, or “circadian rhythms.” These cycles are set by circadian clock genes that are found in nearly every cell in our bodies. The timing of our daily rhythms is strongly influenced each day when light is first detected by our eyes. Light stimulates areas of the brain to tell our bodies to be awake, active, and hungry. When darkness falls, the hormone melatonin is secreted by the pineal gland in the brain and is produced all night long, promoting sleep. Our master clock in the brain controls the timing of our circadian rhythms so they work together: this harmony is important for a healthy functioning body. When our sleep patterns or lightness and darkness cues are severely disrupted—as can happen with night shift work, travel across multiple time zones, or exposure to light during our normal sleeping hours—we can develop what is referred to as “circadian disruption.” Circadian disruption means our circadian rhythms are not working together, which can make us feel ill, increase our risk for poor health, and increase our chances for making mistakes that risk our safety and the safety of others.

The connection between circadian disruption, melatonin, and cancer

Circadian disruption can harm biologic systems that help prevent cancer. For example, in addition to promoting sleep, melatonin can also stop tumor growth and protect against the spread of cancer cells. Laboratory animals whose days and nights were disrupted in scientific experiments had reduced levels of melatonin and increased rates of cancer or tumor growth.

Animal studies have shown that exposure to light at night led to the growth of breast or mammary-gland cancer. Moreover, studies among nurses and other night shift workers showed increased risk of breast cancer that was unexplained by reproductive history, lifestyle factors, body mass index, or socioeconomic status. In most studies, an excess risk of breast cancer was found mainly among women who had worked night shifts for many years or at a high frequency, or who had worked a large number of night shifts over their lifetimes.

What does this mean for workers?

Although we don’t know specific cancer risks potentially associated with night shift work, the NTP and IARC reports both suggest that people who regularly work night shifts appear to be at highest risk. Specifically:

• Night shift work of at least 3 hours between midnight and 5:00 AM
• Frequent night work (3 or more nights per week)
• Long-term night shift work (10 or more years)
• Beginning night shift work in early adulthood (such as before the age of 30)
• Other factors that are more common among night shift workers might contribute to the risk of cancer:
  • Workplace and social stressors
  • Lifestyle behaviors, such as smoking, higher than recommended alcohol consumption, poor diet, not enough physical activity, and being overweight or obese
  • Decreased exposure to sunlight and lower levels of vitamin D

What other health effects are associated with night shift work?

In addition to cancer, night shift work has been associated with type 2 diabetes, heart disease, stroke, metabolic disorders, and sleep disorders (4). Night shift workers might also have an increased risk for reproductive issues, such as irregular menstrual cycles, miscarriage, and preterm birth. Digestive problems and some psychological issues, such as stress and depression are more common among night shift workers. The fatigue associated with nightshift can lead to injuries, vehicle crashes, and industrial disasters (5).

What can night shift workers do to stay healthy?

• Take a NIOSH online training program on shift work to learn about individual and workplace strategies to better cope with working at night.
• Get regular health checkups and tell your doctor if you have any of the following possible symptoms of night shift work:
  • severe fatigue or sleepiness when you need to be awake
  • trouble with sleep, stomach or intestinal disturbances
  • irritability or bad mood
  • poor performance (frequent mistakes, injuries, vehicle crashes, near misses, etc)
  • unexplained weight gain or loss
• Shift workers should practice good health behaviors to combat the effects of night shift work:
  • get enough sleep
  • eat a nutritious diet
  • exercise regularly
  • avoid using tobacco
  • limit alcohol consumption
• Check the NIOSH website periodically for updates on night shift work and health.

Where can night shift workers get help?

• The NIOSH topic page for work and fatigue has many resources to help workers and managers better cope with the demands of shift work and long work hours.
• Since the IARC and NTP reports suggest that night shift workers might be at increased risks of breast cancer and prostate cancer, shift workers should know the risk factors and symptoms for these cancers, and should follow CDC’s screening recommendations for breast cancer and CDC’s screening recommendations for prostate cancer.
• NIOSH has a comprehensive online training program for nurses about shift work and long work hours. It discusses the risks of shift work and long work hours, why we think these occur, and gives workplace and personal strategies to reduce the risks. Although the modules are targeted for nurses, other types of workers and managers could also benefit from taking this training.
• NIOSH has online training for emergency responders who work long hours and online training about fatigue for pilots of small aircraft. NIOSH is developing and testing tailored training programs to help several other types of workers and managers better cope with the demands of shift work and long work hours including law enforcement, taxi drivers, trucking, mining, and manufacturing.
• The NIOSH reproductive health topic pages contain information about reproductive health outcomes associated with shift work.

The National Toxicology Program (NTP) is an interagency program composed of, and supported by, three government agencies within the Department of Health and Human Services, National Institutes of Health, Centers for Disease Control and Prevention (represented by NIOSH) and the Food and Drug Administration.

This blog is also available in Spanish.

Christina C. Lawson, PhD, is a Lead Research Epidemiologist with the NIOSH Division of Field Studies and Engineering.

Elizabeth A. Whelan, PhD, is the Chief of the Field Research Branch in the NIOSH Division of Field Studies and Engineering.

 Tania Carreón-Valencia, PhD, MS, is a Senior Scientist with the World Trade Center Health Program at NIOSH.

Claire C. Caruso PhD, RN, FAAN, is a Research Health Scientist in the NIOSH Division of Science Integration. 

References

1. NTP (National Toxicology Program). 2021. National Toxicology Program Cancer Hazard Assessment Report on Night Shift Work and Light at Night. Available at https://ntp.niehs.nih.gov/go/NSW_LAN
2. IARC Monographs on the Identification of Carcinogenic Hazards to Humans. Vol. 124: Night Shift Work. https://publications.iarc.fr/593
3. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Vol. 98: Painting, firefighting, and shiftwork. Lyon, France. 804 p. http://publications.iarc.fr/116
4. Rosa, D., Terzoni, S., Dellafiore, F., Destrebecq, A. Systematic review of shift work and nurses’ health. [2019] Occupational Medicine, 69(4):237-243.
5. NIOSH [2015]. By Caruso CC, Geiger-Brown J, Takahashi M, Trinkoff A, Nakata A. NIOSH training for nurses on shift work and long work hours. Cincinnati, OH: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication No. 2015-115 (Revised 10/2019). https://doi.org/10.26616/NIOSHPUB2015115revised102019