Shift work and the circadian rhythm

What is 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.

This 24-hour clock inside your body tells you when you’re sleepy or hungry and helps control other physiological processes. The circadian rhythm rises and falls throughout the day and responds to environmental cues like sunlight. It’s why you’re mostly sleepy at night and awake during the day.

If you have a job or schedule that disrupts your circadian rhythm – for example, it forces you to be awake at night and asleep during the day or if you work shifts that frequently change the times you’re asleep and awake – it’s hard on your body and may increase your risk of health concerns, including cancer.

Learn more about how shift work can affect your health and your risk of cancer.

Am I at a greater risk of cancer if I do shift work?

The short answer is: maybe.

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.

Most research has looked at whether shift work increases the risk of breast cancer.

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

  • Having sleep patterns disrupted 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 or change the levels of other hormones, like estrogen.
  • Lifestyle risk factors, including smoking, drinking alcohol, 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 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 happens when these rhythms become out of sync with sleep-wake rhythms.

Tips to reduce your risk from shift work

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.

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 visit find cancer early and talk to your doctor about what’s right for you.