Night-Shift / On-Call Work and Cancer Risk

The number of people working night shifts is increasing every day. Currently, of every 5 working people, 1 works a night shift. According to the results of a survey conducted to find out why people work night shifts, time management and economic reasons lead people to work at night. However, this work schedule can bring an increased risk of certain diseases.

Research suggests that people working evening or night shifts, as a result of changes in their lifestyles and biological patterns, have a higher risk of various chronic diseases such as diabetes, cardiovascular diseases , obesity, and even cancer and certain psychological disorders than people working normal hours.

The key mechanism underlying shift work and the increased risks of some diseases is thought to be circadian rhythm (the biological clock) . In brief, circadian rhythm is a mechanism regulating all bodily activities, such as metabolism and energy balance, protein and hormone production, and sleep requirements. Circadian rhythm mechanisms regulated by genes can be described both at the physiological level in the brain and at the cellular level.

The circadian clock affects our body's physiology in many ways: This clock helps regulate sleep patterns, eating behavior, hormone secretion, blood pressure and body temperature. A large proportion of our genes are regulated by the clock.

- Related topic: Why was the 2017 Nobel Prize in Medicine awarded to circadian rhythm again?

Circadian rhythm can be affected by external stimuli. This is why people working night shifts are thought to have a higher risk of certain diseases. Working at a time when the circadian rhythm calls for sleep can disrupt certain balances in the body, and some diseases may develop as a result. The increased risk of developing diabetes in people working night shifts, is more readily explained by research, but the situation differs for cancer, because the relationship between night shifts and cancer is more complex than the relationship with other diseases. Scientists therefore wondered how cancer incidence rates among people working late hours compared with those working normal hours, and studies were carried out to understand this relationship.

As a result of studies on the relationship between shift work and cancer, in 2007 the International Agency for Research on Cancer (IARC) classified night shifts as probably carcinogenic (Group 2A) because they disrupt circadian rhythm.

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After 2007, a group of British researchers investigated the relationship between night shifts and breast cancer in particular . The study, whose results were announced in 2016, included, from 10 different studies, a total of 1.4 million women. Ultimately, compared with women who did not work night shifts, those who did - even for 30 years - did not have a higher risk of breast cancer. However, because this research covered only breast cancer, it could not support the conclusion that "night shifts do not cause cancer". Another study was carried out in mice. In this study, the mice were made to work in 12 hours of light and 12 hours of darkness. Mice kept awake in the dark developed lung adenocarcinoma, liver cancer and malignant melanoma . Another study observed an association between prostate and colorectal cancer and night shifts, but the studies were insufficient to prove this. At the IARC 2019 meeting, night-shift work continued to be classified as a Group 2A carcinogen for humans.

Night-shift work, as a definite cause of cancer in humans (Group 1 carcinogen), cannot be classified mainly because of the presence of many environmental factors, such as increased smoking and irregular eating during night shifts. For this reason, it is not currently possible to conclude that night shifts directly cause cancer. Nevertheless, this is certainly a topic worth investigating and clarifying, and advances in biotechnology may allow more definitive conclusions in the future.