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Does Night Shift Work Increase the Risk of Breast Cancer?

Medically reviewed by Todd Gersten, M.D.
Written by Amy Isler, RN
Posted on November 29, 2021

Working the night shift can have many perks, including higher pay and less traffic. However, studies have shown that women who work night shifts have a 5 percent to 20 percent higher risk of developing breast cancer. The increased risk relates to the stress a person’s body endures from disrupted sleep and overexposure to artificial light.

A MyBCTeam member noticed that there were a lot of nurses active on the site and wondered if there might be a link: “Is it me or are there a lot of nurses on this site? Are we just good about getting mammograms? Worked the night shift? Or have personalities that deal with stress a certain way? It is very interesting.”

Currently, 1 in 8 women in the U.S. will develop some form of breast cancer in their life, according to the American Cancer Society. Those that work the night shift are at a higher risk for both invasive breast cancer and noninvasive ductal carcinoma in situ (abnormal cells in the milk ducts of breasts). The risk is even greater for those with other risk factors including a family history of breast cancer, obesity, age (55 and older), alcohol consumption, smoking, and decreased physical activity. The amount of time someone spends working the night shift is also a factor: According to one study, five years of night shift work adds a 3 percent increased risk of developing breast cancer.

This article discusses why night shift work is a breast cancer risk factor and what steps you can take to decrease your risk while working during the night.

How Does Night Shift Work Increase Breast Cancer Risk?

The normal sleep/wake cycle for humans is to be active during the day and to sleep during the night. A disruption to a person’s circadian rhythm (natural sleep cycle) can contribute to negative health outcomes including:

  • Cardiovascular disease
  • Obesity
  • Gastrointestinal (GI) disturbances
  • Anxiety/depression
  • Diabetes
  • Chronic fatigue
  • Endocrine disorders
  • Risk of cancer (specifically breast cancer)

Circadian rhythm disruption can also increase a person’s risk of being in a car accident — or making dangerous errors at work that can result in serious consequences, especially for those who work in health care. One study found that nurses affected by sleep disruption made more errors related to administering medications.

“I’m a nurse working night shifts,” shared one MyBCTeam member. “At my previous employer there were five nurses with breast cancer in our department.”

Systematic review studies have shown that artificial light — and the hormonal disruptions it can trigger — is the root cause of circadian rhythm disruption. This increases a person’s risk of developing breast cancer working the night shift.

Artificial Light and Hormones

The artificial light to which night shift workers are exposed can cause confusion in their sleep patterns. Specifically, it decreases the production of melatonin, a hormone that is triggered during darkness to help us sleep. Melatonin also has the important function of helping stop tumor growth.

Getting insufficient melatonin due to exposure to artificial light can trigger a dangerous cancer pathway, which can result in tumor growth in the breast. Because of these findings, the International Agency for Research on Cancer concluded that shift work that involves circadian disruption is probably carcinogenic to humans. It is important to note that only night shift and rotating shift work were found to increase a person’s chances of developing breast cancer, not shift work in general.

The circadian rhythm also affects glucocorticoids, specifically the hormone cortisol. Cortisol is responsible for helping keep the immune system healthy by fighting off dangerous pathogens. The human body produces the greatest amount of cortisol in the morning, right before a person wakes up. This helps signal the body that it is time to get up.

When the glucocorticoid cycle is disrupted by disturbances to the sleep cycle, the body responds by increasing cortisol levels. This can foster an environment suitable for estrogen receptor-negative tumor growth linked to breast cancer.

A MyBCTeam member made the connection between breast cancer and stress: “Teachers don't have to work night shifts like nurses do, but I do believe stress plays a factor in how our bodies function.”

Managing Night Shift Risk

From an epidemiological and public health perspective, preventative measures and changes to shift system policies can help keep workers safe. These changes might include:

  • Limiting the number of postmenopausal individuals who can work the night shift
  • Ensuring workers receive an appropriate number of breaks
  • Providing healthy and ergonomic working environments
  • Encouraging routine cancer screenings for night shift workers
  • Educating employees on the health risks of working the night shift

For those who are currently working the night shift or plan to in the future, creating positive health habits and routines can help minimize the health risks of sleeping during the day. Some tips to consider, provided by the Centers for Disease Control and Prevention (CDC), include:

  • Taking the National Institute for Occupational Safety and Health’s course Training for Nurses on Shift Work and Long Work Hours
  • Getting regular health checkups and routine screenings
  • Practicing good sleep hygiene
  • Eating a nutritious diet
  • Exercising regularly
  • Avoiding alcohol and tobacco
  • Doing night shift work on only a short-term basis, if possible

You Are Not Alone

MyBCTeam is the social network for people with breast cancer, survivors, and their loved ones. More than 54,000 members with experience in breast cancer gather to ask questions, give advice, and share about their experiences with others who understand.

Do you work the night shift? What other health challenges have you faced from working during the night? Share your experience in the comments below or on MyBCTeam.

  1. Are Women Who Work Night Shifts at a Higher Risk for Developing Breast Cancer? — National Center for Health Research
  2. How Common Is Breast Cancer? — American Cancer Society
  3. Invasive Breast Cancer (IDC/ILC) — American Cancer Society
  4. A Meta-Analysis on Dose-Response Relationship Between Night Shift Work and the Risk of Breast Cancer — Annals of Oncology
  5. Night Shift Work — A Risk Factor for Breast Cancer — International Journal of Environmental Research and Public Health
  6. Effect of Circadian Rhythms and Driving Duration on Fatigue Level and Driving Performance of Professional Drivers — Journal of the Transportation Research Board
  7. Estimating the Circadian Rhythm in the Risk of Occupational Injuries and Accidents — Chronobiology International
  8. Impacts of Nurses’ Circadian Rhythm Sleep Disorders, Fatigue, and Depression on Medication Administration Errors — Egyptian Journal of Chest Diseases and Tuberculosis
  9. Carcinogenicity of Shift-Work, Painting, and Fire-Fighting — The Lancet
  10. The Pathophysiologic Role of Disrupted Circadian and Neuroendocrine Rhythms in Breast Carcinogenesis — Endocrine Reviews
  11. Recent News About Night Shift Work and Cancer: What Does It Mean for Workers? — Centers for Disease Control and Prevention
  12. NIOSH Training for Nurses on Shift Work and Long Work Hours — Centers for Disease Control and Prevention
  13. Sleep Hygiene — Sleep Foundation
  14. Night-Time Shift Work and Related Stress Responses: A Study on Security Guards — International Journal of Environmental Research and Public Health
  15. Stress: The Role of Glucocorticoids —

Posted on November 29, 2021
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Todd Gersten, M.D. is a hematologist-oncologist at the Florida Cancer Specialists & Research Institute in Wellington, Florida. Review provided by VeriMed Healthcare Network. Learn more about him here.
Amy Isler, RN is a registered nurse with over six years of experience as a credentialed school nurse. Learn more about her here.

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