OTHERS – PROBABLE CARCINOGEN (IARC 2A)
IARC Monograph Vol. 124, 2019 (Group 2A), in prep.
- Work time organized to cover more than the usual 8-hour workday, up to a 24-hour period
- Associated cancers: Limited evidence for breast, colorectal, and prostate cancer
- Occupational exposures: Approx. 1.8 million Canadians work shifts that fall between midnight and 5am, primarily in trades, accommodation and food services, manufacturing, and health care and social assistance
- Fast fact: Shiftwork at night disrupts the internal biological clock (circadian rhythm). It suppresses melatonin production, disrupts sleep patterns and food digestion, and de-regulates genes involved in cancer pathways.
Shiftwork can be defined as the organization of working time by different teams in succession to cover more than the usual eight hour work day, up to a 24 hour period. Some people work shiftwork on rotation while others perform regularly scheduled day, evening, or night shifts.
Shiftwork outside of regular daytime hours results in a mismatch between the body’s circadian rhythms and the solar light-dark cycle. Circadian rhythms generate the sleep-wake cycle in humans, acting as an internal biological ‘clock’. Circadian rhythms are synchronized with environmental factors, particularly the 24-hour solar light-dark cycle.[2,4] Shiftwork at night has been found to be the most disruptive to internal circadian rhythms. Circadian disruption is known to suppress melatonin levels, de-synchronize rhythmic body processes (i.e. sleep patterns and food digestion), and de-regulate genes involved in cancer pathways.[1,5]
The International Agency for Research on Cancer (IARC) has classified night shiftwork as Group 2A, probably carcinogenic to humans, based on sufficient evidence of cancer in animals and limited evidence in humans. In several well-designed animal studies, alteration of rodents’ light-dark schedules (e.g. exposure to repeated 8 hour time shifts in the light-dark schedule, or exposure to a stable 12 hour light and 12 hour dark schedule) lead to increased rates of cancer or tumours . Epidemiological studies observed an increased risk of breast cancer among those working high-intensity shifts over long durations compared to those who do not work at night. However, findings were inconsistent across studies. Some studies considered in the IARC review suggest that prostate and colorectal cancer may also be associated with night shiftwork. The findings from the epidemiological studies are somewhat limited by factors such as potential confounding and inconsistent definitions of shiftwork. The Group 2A classification was further supported by strong mechanistic evidence that alteration of light-dark schedule among rodents leads to immunosuppression, chronic inflammation, and cell proliferation, all of which may contribute to cancer growth.
In addition to potentially carcinogenic outcomes, shiftwork has been associated with changes in mental and physical performance at work, as well as fatigue, stress, disruption to family and social life, depression, and anxiety.[1,7,8] Digestive disorders such as indigestion, heartburn, nausea and loss of appetite, and cardiovascular disorders such as hypertension have been observed in shiftworkers. Shiftwork has also been linked to reproductive health problems in women and aggravation of previous health conditions such as asthma, diabetes, and epilepsy.
Regulations and Guidelines
No occupational exposure limits for shiftwork were located for Canada. In Europe, EU Council Directives ensure that member states take measures concerning night shift length, average working times (daily and weekly), and rest periods (daily and weekly). The International Labour Organization has also issued recommendations around night work frequency, shift length, and rest periods. Both of the above pay special attention to pregnant and young workers.
Shiftwork in Canada
CAREX Canada estimates that approximately 1.8 million Canadians work shifts between midnight and 5am. This represents around 12% of working Canadians. Industries with the most workers working at night include trades, health care and social assistance, manufacturing, and accommodation and food services.
For detailed estimates of exposure to shiftwork, see the occupational exposures tab.
- Ontario Institute for Work and Health. Shift work and health (2012)
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