About Shiftwork

Shiftwork is work scheduled outside of regular daytime working hours. Night shiftwork, occurring during regular sleeping hours (approximately 12 a.m. to 5 a.m.), is most associated with harmful health effects. This may be due to disruption of the body’s natural sleep-wake cycle.

Night shiftwork is classified as a probable carcinogen by the International Agency for Research on Cancer (IARC 2A), based on limited evidence that it may cause breast, prostate, and colorectal cancers [1]. It can also cause other health effects such as fatigue, stress, depression, anxiety, heartburn, nausea, and loss of appetite. CAREX Canada estimates that approximately 1.8 million Canadians work shifts between 12 a.m. and 5 a.m. [2].

Burden of Cancer Results

Approximately 470-1,200 suspected female breast cancers may be due to shiftwork each year, based on 2011 cancer statistics [3]. This amounts to 2 to 5% of all female breast cancers diagnosed annually in Canada. These results are shown as a range because research studies are not in agreement on the impact of shiftwork on breast cancer risk.

Most shiftwork-related breast cancers occur among workers in the healthcare industry (see pie chart).

Exposure Reduction

Completing work during standard daylight hours is the best way to avoid the health effects associated with night shiftwork. However, this is often not practical, as night work is necessary to maintain essential services and continuous processes in many industries. It is not clear what other preventative measures are most effective in reducing the carcinogenic effects of shiftwork. However, better shift scheduling, such as flexible work schedules and rotating schedules that move quickly from morning to afternoon to evening shifts, can minimize some of the negative health impacts [4,5,6]. More research is needed to understand the impact these and other strategies can have on future cancer burden.

Burden Resources

Fact Sheets

Research Article

Related OCRC Projects

Related Research Articles/Fact Sheets

Resources from OCRC Workshops


  1. International Agency for Research on Cancer Vol 124 Working Group. Carcinogenicity of night shift work. Lancet Oncology 2019;20(8):1058-1059.
  2. CAREX Canada. Shiftwork Occupational Exposures Profile.
  3. Pahwa M, Labrèche F, Kim J, Harris MA, Song C, Peters CE, Arrandale VH, Davies H, McLeod CB, Demers PA. The impact of night shift work on breast cancer: results from the Burden of Occupational Cancer in Canada StudyAm J Ind Med. 2019 Aug;62(8):635-642.
  4. Cancer Care Ontario. Cancer Fact: Shiftwork, Breast Cancer and Health: Interventions to Reduce the Risks.
  5. Neil-Sztramko SE, Pahwa M, Demers PA, Gotay CC. Health-related interventions among night shift workers: a critical review of the literature. Scand J Work Environ Health. 2014;40(6):543-556. 
  6. Canadian Centre for Occupational Health and Safety. OHS Answers: Rotational Shiftwork.