shift work disorder carcinogen cancer sleep health asaa american sleep apnea associationOur “Did You Know…?” series features discussions about sleep health as it relates to comorbid conditions.
[Previously, we discussed sleep and pulmonary hypertension.]
What are comorbid conditions? These are the medical illnesses, issues, or diagnosed conditions which frequently occur with other kinds of illnesses, issues, or conditions. The reason they are comorbid is that they share many of the same underlying causes and/or symptoms.
Because sleep is a whole-body process, poor sleep frequently accompanies more than one health problem, making it a condition which is comorbid to other medical conditions.

The connection between shift work disorder and cancer

The time of day when you sleep factors prominently in the overall health of your sleeping life. A third of each day is (or should be) dedicated to getting sleep.

Our circadian rhythms typically assign sleep as our key nighttime “activity.” This is because our bodies switch into a separate gear overnight. This change in pace follows a pre-established regimen assigned by the body clock. That regimen includes cellular repair and a good “sweeping out” of the brain, as well as a boost to our immune system.

However, if you work odd hours, night shifts, rotations, or graveyard, you run the risk of developing what is known as shift work disorder. If this happens, you’re not likely to enjoy as many of these built-in benefits.

What’s even worse: Since December 2007, following a decade of research, the International Agency for Research on Cancer and the World Health Organization designated shift work as a Class 2A carcinogen: This means SWD is “probably carcinogenic to humans.”

[This designation is generally used when proof of carcinogenicity in humans is limited. However, there still exists sufficient evidence of carcinogenicity in lab experiments using animals.]

What is shift work disorder?

Shift Work Disorder (SWD) (sometimes referred to as SWSD, or Shift Work Sleep Disorder) is defined as a sleep-wake disorder caused by disruption to the circadian rhythm. It happens as the result of remaining wake (on shift) during a period of time which overlaps with one’s ordinary sleep schedule.

When SWD occurs, people who suffer from it typically experience insomnia at night and excessive daytime sleepiness.

SWD directly causes chronic sleep deprivation. People who work these schedules never catch up on necessary sleep. Over time, they develop significant and hard-to-reverse “sleep debt.”

When people develop this level of chronic sleep loss, it leads to negative risks for developing chronic illness. These include cancer, weight gain, heart disease, diabetes, memory problems, a weakened immune system, mood disorders, stroke, hypertension, and cognitive dysfunction.

People with SWD also risk a higher incidence of workplace accidents, dangerous mistakes in judgment, and driver fatigue.

Who suffers from shift work disorder?

Industries which employee shift workers, or expect salaried workers to work beyond the 9 to 5 work day, include the following:

  • Restaurants and hotels
  • Hospitals and urgent care clinics
  • Factories and industrial plants
  • Construction sites
  • Shipping and delivery
  • Civil servants and first responders
  • Cleaning services
  • Call centers and technical service providers in IT
  • Transportation (public, private, commercial)
  • Casinos and other entertainment venues
  • Home healthcare and nursing facilities
  • Global corporate management
  • Filmmakers and news media
  • Security services

If you think about it, these industries employ a lot of people. According to the French-based International Agency for Research on Cancer (IARC), almost 20 percent of the workforce just in Europe and North America alone engages in shift work.

“Shiftwork (sic) is most prevalent in the health-care, industrial, transportation, communications, and hospitality sectors,” the IARC said in a Reuters article in 2007.

Some research suggests that 10 percent of all shift workers develop SWD; check out this fact-filled report, “Shift work and Sleep,” published by the Centers for Disease Control and Prevention in 2016.

Relationships between shift work disorder and cancer

When we don’t get enough sleep, this affects hormones like cortisol and melatonin. These hormones are critical for helping us wake up in the morning or fall asleep at night. They also influence the behaviors of cancer cells and assist in signaling the immune system of their presence.

When we run short of melatonin, our body compensates by increasing more estrogen. Unfortunately, higher estrogen levels in the body are a known risk factor for breast cancer. Female night shift workers are a high-risk group for this form of cancer. Meanwhile, male night shift workers have higher rates of prostate, bowel, lung, and bladder cancer.

It’s not so much the cancer that’s the problem. It’s the failure of the immune system to suppress the proliferation of cancer cells. This leads to tumors and the spread of the disease. The American Cancer Society offers great advice for getting better sleep as a preventive or defensive exercise against cancer.

Short sleep, short life?

Sleep truly is a critical process for supporting our body’s built-in process (the immune system) for preventing disease. Yet, as a society, we don’t connect good sleep with good health. However, we should.

Just last month in TreeHugger, the blog author urged “It’s time to take sleep seriously.” They followed with a feature on neuroscientist and author Matthew Walker, who recently published the book, Why We Sleep: The New Science of Sleep and Dreams.

Walker has researched sleep for more than 20 years. He directs the center for Human Sleep Science at the University of California, Berkeley. In his book and elsewhere, he urges everyone to acknowledge the very strong and science-backed connection between lack of sleep and disease.

According to the blog post at TreeHugger, “more than 20 large-scale epidemiological studies” mirror Walker’s conclusion that “the shorter your sleep, the shorter your life.”

As sleep deprivation specifically relates to cancer, Walker said:

“…after just one night of only four or five hours’ sleep, your natural killer cells—the ones that attack the cancer cells that appear in your body every day—drop by 70 percent” and “a lack of sleep is linked to cancer of the bowel, prostate and breast.”

What he’s talking about is that domino effect that takes place when circadian rhythms are disrupted. Our systems run off kilter when this happens, especially the one designed to protect us: the immune system.

So what do we know about specific groups of people we know to be shift workers?

Nurses studies

In the August edition of the American Journal of Epidemiology, an article shared how researchers performed a major study using data from two major cohort studies—the Nurses’ Health Study (data captured from over 78,000 subjects between 1988 and 2012, and the Nurses’ Health Study II (sampling from nearly 115,000 subjects between 1989 and 2013).

The purpose of the study was to identify associations between shift work among nurses and breast cancer risk.

The result? “[L]ong-term rotating night-shift work was associated with a higher risk of breast cancer, particularly among women who performed shift work during young adulthood,” according to the study authors.

A third nurses study

Harvard research appeared just two weeks after the night-shift study conducted and published in the AJE. In Environmental Health Perspectives, they revealed that female subjects who were shown to be exposed to large amounts of artificial light could be 14 percent more likely to develop breast cancer, with those who work night shifts particularly at risk.

They approached data from the Nurses’ Health Study II with data from satellite images taken at night linked to the home addresses of each study participant to arrive at their findings.

The most likely candidates for developing breast cancer? The top fifth of subjects with the highest light exposures.

“Probably carcinogenic”

Much of the challenge in understanding the relationship between circadian disruption and cancer is finding the best ways to measure correlations in order to identify cause.

Since the WHO’s “probably carcinogenic” finding, much has been argued about whether that’s true, given a shortage in specific kinds of research.

As recently as October 2016, a major Oxford research study concluded that classifying SWD as a cause for breast cancer could no longer be justified. Their findings in a very large study seemed to conclude night shifts were not carcinogenic.

Meanwhile, this article in Hazards suggests that the Oxford research, which said the WHO’s research had been flawed, may be, in itself, flawed.

How can one be certain that working at night will actually lead to breast cancer, if human studies are still inconclusive and researchers are in disagreement?

5 things you can do

Until we can better (and more confidently) identify the links between shift work disorder and cancer, we should prioritize our precious hours of sleep.

Just because so many of us work odd hours doesn’t necessarily mean we’re necessarily doomed. Researchers could be partially wrong (or right), after all.

Until the research can be more conclusive, here are a few ways to reclaim lost sleep to offset any risks for developing cancer or any other concern.

  1. Use room-darkening window treatments in your bedroom. The darker your sleeping space, the more likely you will achieve recovery sleep. If shades or curtains are out of the question, try wearing a sleep mask.
  2. Use a “do not disturb” system in your home. Family should be apprised of your sleep schedule during the day. Try to explain that they need to honor and respect your need for sleep; it’s for your health.
  3. Turn off your cell phone and your doorbell. Wearing earplugs or using a white noise machine to “cover” ambient noise can help keep arousals to a minimum.
  4. Wear sunglasses on your way home from an overnight shift: This can fool your body into thinking it is still dark out. You may better achieve sleep once you’re home.
  5. Stay on schedule: Keep to the same bedtime and rise time every single day, even on your days off. If that’s too extreme a measure, allow a “recovery day” to correct your rhythms if you work several days on, then have several days off. If you work an extremely fragmented schedule of shifts, work with your coworkers and employer to rebuild the schedule that you (and your peers) can live and work with.