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Shift work and the circadian rhythm

The circadian rhythm is the body’s internal clock. It is a built-in biological rhythm, a rough 24-hour cycle that regulates certain physiological processes such as sleeping and eating. The circadian rhythm can be adjusted to environmental cues, such as daylight.

  • Shift work, the circadian rhythm and cancer

    The International Agency for Research on Cancer (IARC) has concluded that shift work (working outside the normal workday, often at nighttime) probably causes cancer when it affects circadian rhythms (our internal clock). This conclusion is based on data from animal studies and a limited amount of research in humans. Find out more about how cancer-causing substances are classified.

    Researchers are not sure how shift work increases a person’s risk of getting cancer, but some possible explanations that are being studied are:

    • Disruption of sleep patterns can change the body’s natural rhythms or affect the production of chemicals.
    • Light exposure at night can decrease the amount of melatonin (a hormone) produced by the brain. Melatonin may slow tumour development and reduce the levels of certain reproductive hormones, like estrogen.
    • Lifestyle risk factors, including smoking, alcohol use, an unhealthy diet, lack of exercise and an unhealthy weight, may be a result of shift work.
    • Lower exposure to daylight may decrease the body’s production of vitamin D or change the body’s production of hormones.
    • Phase shift may contribute to cancer development due to changes at the cell and tissue level that result from working at unusual times. Some organs or cells within the body have a rhythmic process that is synchronized to the sleep-wake cycle. Phase shift occurs when these rhythms become out of sync with sleep-wake rhythms.
    Possible link to breast cancer

    Researchers are mainly interested in the link between breast cancer and working night shifts because research results aren’t conclusive about a link between shift work and other cancers. The Nurses’ Health Study in the US shows that women who worked on night shifts for more than 20 years, with at least 3 night shifts per month, have an increased risk of breast cancer. Researchers believe exposure to artificial light at night can affect the brain’s production of the hormone melatonin and can cause the ovaries to produce an increased amount of the hormone estrogen. High levels of estrogen in the body may increase the risk of breast cancer.

    Occupational risk

    Workers on the night shift for 20 years or more may have an increased risk of getting cancer. This may include people working in healthcare, protective services, industrial manufacturing, mining, transportation, communication, leisure and hospitality.

    In March 2009, Denmark was the first country to give financial compensation to Danish women who had worked night shifts for at least 20 years and then been diagnosed with breast cancer. The Society will continue to monitor this issue, as well as how this decision affects employment standards internationally.

  • Tips to reduce your risk from shift work

    Workers doing night shifts should make healthy lifestyle choices that include eating well, being active, maintaining a healthy body weight, not smoking, limiting alcohol consumption and getting enough vitamin D.

    Screening is also an important part of overall health, even when you feel well. Please read our screening information and talk to your doctor about what’s right for you.

  • Suggested links for more information
    General information about shift work

    This fact sheet provides information about the potential health risks associated with shift work, industries where shift work is most prevalent, and statistical data regarding the impact of shift work on Canadian workers.

    CAREX Canada – Carcinogen profile: shift work

    Science-based evaluations of shift work

    This website discusses the scientific evidence regarding the links between shift work and cancer.

    International Agency for Research on Cancer – Evaluation of shift work



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