Shiftwork can be defined as the organization of working time by different teams in succession to cover more than the usual 8-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.
Circadian rhythms generate the sleep-wake cycle in humans, acting as an internal biological ‘clock’. Endogenous circadian rhythms are synchronized with environmental factors, particularly the 24-hour solar light-dark cycle.[10,11] Shiftwork outside of regular day time hours results in a mismatch between the body’s circadian rhythms and the solar light-dark cycle. Shiftwork at night has been found to be the most disruptive to internal circadian rhythms. Circadian disruption is characterized by melatonin suppression, de-synchronization of rhythmic body processes (i.e. sleep patterns and food digestion), and de-regulation of genes involved in cancer pathways.[1,2]
IARC has classified shiftwork with circadian disruption as Group 2A, probably carcinogenic to humans, based on sufficient evidence in animals and limited evidence in humans. Most studies investigating disruption of circadian rhythms in rodents showed a significant increase in tumour incidence following exposure to constant light, dim light at night, or simulated chronic jet lag. Epidemiological studies have observed an increased risk of breast cancer in long-term night shiftworkers when compared to people not working at night.[2, 12] In particular, two cohort studies of nurses[5,6] and one of flight attendants found moderate increases in the risk of breast cancer. In addition to elevated risk of breast cancer, a recent meta-analysis found that shiftwork is associated with ovary cancer and melanoma. These findings are somewhat limited by factors such as potential confounding and inconsistent definitions of shiftwork.
In addition to carcinogenic outcomes, shiftwork has been associated with changes in mental and physical performance at work, fatigue, stress, disruption to family and social life, depression and anxiety.[1,8,9] 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 be linked to reproductive health problems in women and aggravation of previous health conditions such as asthma, diabetes and epilepsy.
No occupational exposure limits for shiftwork in Canada were located. 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 (ILO) 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 1.9 million Canadian workers have work shifts between midnight and 5am. This represents around 13% of working Canadians. Canadian industries with the most workers working at night include manufacturing, trades, health care and social assistance, and accommodation and food services. For detail CAREX Canada estimates for exposure to shiftwork, see our prevalence and exposure level estimate pages.
Our team has performed a detailed scan of exposure control resources and assembled a compilation of key publications and resources. These are organized by type of exposure (environmental or occupational) and by specificity (general or carcinogen-specific). Please visit our Exposures Reduction Resources page to view.
We also recommend exploring the Prevention Policies Directory, a freely-accessible online tool offering information on policies related to cancer and chronic disease prevention. Providing summaries of the policies and direct access to the policy documents, the Directory allows users to search by carcinogen, risk factor, jurisdiction, geographical location, and document type. Click here to learn more about policies specific to shiftwork in the Directory. For questions about this resource, please contact a member of the Prevention Team at the Canadian Partnership Against Cancer at email@example.com.
Stevens RG, et al. Considerations of circadian impact for defining ‘shift work’ in cancer studies: IARC Working Group Report. Occup Envion Med 2011;68:154-162
Straif K, et al. Carcinogenicity of Shift Work, Painting and Firefighting Lancet Oncology 2007;8(12):1065-66