What is night shift work?
Night shift work involves work done during the regular sleeping hours of the general population, and night shift workers tend to have critically important work duties: surgical and medical care, law enforcement, transportation, supply chain, and maintaining the power grid.
Without night shift workers, society as we know it would no longer exist. During the night, we would not have electricity, security, or medical care. During the day, we would notice serious supply chain issues, overcrowded airports, dangerous electrical problems, oil spills, crowded emergency rooms, and many unnecessary deaths.
But shift work is hazardous to shift workers themselves. Working the night shift just three days per month has been linked to an increased risk of death from all causes, including cancer, heart disease, dementia, diabetes, heart attack, stroke, nonalcoholic fatty liver disease (NAFL), and accidents. In women, night shift work has been linked with chronic miscarriage, infertility and early menopause.
Please note: Throughout this article, I may use the terms night shift work, night work, and shift work interchangeably. All of these refer to work done outside normal daytime hours (when most people are sleeping).
Cancer
In June 2019, a Working Group of 27 scientists from 16 countries met at the International Agency for Research on Cancer (IARC) in Lyon, France to assess available research on night shift work and its effect on cancer risk. Based on the preponderance of the evidence, the Working Group decided to classify night shift work as a Group 2A carcinogen, which means it is "probably carcinogenic to humans."
Much of the research on the potential cancer-causing risks of night shift work comes from the Nurses' Health Study 1 and Nurses' Health Study 2. These studies found that working night shift at least three nights per month was associated with an increased risk of several different types of cancers, including breast cancer and colorectal cancer. Working night shift at least three times per month for one to 15 years was associated with a greater risk of cancer than not working night shift at all or for less than one year, and working night shift for more than 15 years was associated with the highest cancer risk.
A meta-analysis published in 2020 found that working night shift at any point during a career was associated with a significantly increased risk of melanoma (a dangerous form of skin cancer).
Dementia
There has also been a considerable amount of research linking night shift with dementia. A Danish Nurse Cohort study, also published in 2020, found that nurses who worked occasional night shifts for longer than six years had a higher incidence of dementia than those who had worked night shifts for less than one year.
A 2018 study based on data from the Swedish Twin Registry and the Screening Across The Lifespan (SALT) sample found that individuals who had ever worked shift work or night work had an increased incidence of dementia. A modest dose-response association was observed, meaning that people who had worked shift work or night work for many years had an even higher incidence of dementia. Those who had worked more than 20 years doing shift work or night work had the highest incidence of dementia.
Diabetes
A study from data in the UK Biobank (which includes a total of 272,214 people) found that rotating shift work is associated with a higher incidence of Type 2 diabetes. A dose-response association was also observed.
This finding was not surprising since numerous other studies had already found an association between type 2 diabetes and night shift work. Previous studies include a meta-analysis from 28 individual reports and a total of 226,652 participants that found that any history of night shift work increased Type 2 diabetes risk. A subgroup analysis found that there was a stronger association between night shift work and type 2 diabetes for men than for women.
Non-Alcoholic Fatty Liver
An increasing amount of evidence also suggests that night shift work may trigger liver dysfunction. A study published in 2019 that involved 4,740 workers, of which 39.5% of them were night shift workers, found that the night shift workers had an increased risk of alanine transaminase (ALT) being elevated above 40 U/L.
In 2020, a study among steelworkers in China found a link between night shift work and nonalcoholic fatty liver disease (NAFLD) in men. Working more than seven night shifts per month or an average night shift length of more than eight hours was associated with the higher incidence of NAFLD than working fewer than seven night shifts per month with an average night shift length shorter than eight hours, but workers who had any history of night shift work were more likely to have NAFLD than those who had never worked night shift.
Cardiovascular Disease
The Nurses' Health Studies followed 189,158 initially healthy female nurses over the course of 24 years. Nurses who worked a rotating night shift at least three nights per month had a higher incidence of cardiovascular issues such as nonfatal heart attack, heart attack, coronary artery bypass graft surgery, stents, and angioplasty than nurses who had no history of rotating night shift work. Those who worked five to nine years on a rotating night shift had a higher risk of heart disease than those who worked fewer than five years on a rotating night shift, and those who worked 10 or more years on a rotating night shift had the highest risk.
Night shift work increases risk of cardiovascular disease in men too. A study on factory workers in Norway found that rotating shift work including day and night shifts lasting up to 12 hours was linked to an increased risk of coronary heart disease and stroke, and a systematic review and meta-analysis involving 173,000 individuals reported an increased risk for cardiovascular disease among shift workers that increased by 7.1% for each additional five years as a shift worker.
Prevention
This information may sound grim. However, everything we do involves some amount of risk. Knowing what your potential risks are is the first step in mitigating those risks.
In next month's issue, we'll learn about circadian clocks in the body and explore some of the strategies that night shift workers can follow in order to reduce their potential health risks.
Questions or comments? Email [email protected] or go to DrJamieKoonce.com.
Author Bio
Dr. Jamie Koonce, DACM, L.Ac., Dipl.OM offers a health optimization program that is tailored to your specific health needs and goals. This may include a combination of laboratory testing, nutrition and lifestyle recommendations, herbal medicinals, nootropics, and acupuncture. Dr. Jamie is also the Founder & CEO of Good Glucose, Inc., a tech startup that is combining AI and gamification with continuous glucose monitoring in order to help individuals avoid the top 10 causes of morbidity and mortality in the United States. Learn more at GetGoodGlucose.com.