Nordic research finds new associations between shift work and health and gives recommendations on how to organize shift work

New studies from a large NordForsk consortium confirm that night shift work is associated with increased risk for serious health risks like accidental injuries and sickness absence. The researchers give recommendations on how to organize shift work in order to minimise health and safety risks.

Press release 44/2020, WOW Symposium, Espoo Finland

About 20% of all employees in Europe are shift workers. A large NordForsk-funded project (Working Hours, Health, Well-being and Participation in Working Life, WOW) produced several practical recommendations on good shift ergonomics due to utilizing of large registry-based data of working hours and health in the Nordic countries (1,2). The data includes, e.g., information on daily working hours in the social and health care sector. “For the first time, we are able to utilize dose-response data on the association of different work characteristics with health making it possible to give much more detailed recommendations on optimal work characteristics in shift work.”, emphasizes professor Mikko Härmä.

New evidence on the association of shift work with breast cancer, maternal health and miscarriage

Several WOW studies have studied the association of shift work with cancer. An international Working Group convened by the International Agency for Research on Cancer (IARC) under WHO classified in 2019 that night work is probably carcinogenic to humans and may cause breast, prostate and colo-rectal cancer(3). Several WOW researchers joined the extensive review published this year.

“The existing high-quality studies on night shift work and cancer show that a higher number of night shifts per week or long duration is associated with increased breast cancer risk. On the other hand, studies with low intensity and/or short-duration of night shifts shown no notable increased risk.” confirms Dr Johnni Hansen from Danish Cancer Society Research Center.

The large nationwide Danish payroll data based study of hospital working women showed an increased risk of miscarriage in women who had two or more night shifts the previous week, compared with women who did not work night shifts (4). The accumulated number of night shifts during weeks 3–21 of pregnancy increased the risk of miscarriage in a dose-dependent pattern.

“To reduce miscarriage, pregnant women should not work more than one night shift in a week”, says professor Anne Helene Garde from the National Research Centre for the Working Environment in Denmark.

Occupational injuries and sickness absence depend on work scheduling practises

The association of work shift characteristics with occupational injuries has been studied in Denmark, Norway and Finland. A Danish study showed that the risk of injury was higher during the evening and night shifts (5).  Another study showed that the shorter the time between shifts, the higher the risk of injury (6). In a Norwegian study, an increase in the number of quick returns (as defined by time periods with less than 11 hours between the shifts) was associated with a higher risk of self-reported work-related accidents, near-accidents and dozing off at work (7).

“In the Finnish study (8), we also found that the risk for occupational injuries became higher in 12 hour or longer work shifts” says professor Mikko Härmä from the from the Finnish Institute of Occupational Health.

WOW studies have also investigated the association of shift characteristics with sickness absence. A Finnish study showed that short sickness absences of 1–3 days were more frequent after long working hours, several consecutive night shifts and frequent quick returns (9). The most recent Danish-Finnish study on long sickness absences (30+ days) indicate that evening work and five or more consecutive night shifts are associated with higher risk of long-term sickness absence, especially among the older employees (10).

“The results indicate that the scheduling of working hours is likely to affect risk of long-term sickness absence” tells professor Anne Helene Garde from the National Research Centre for the Working Environment in Denmark.

WOW key recommendations on scheduling of shift work

Researchers recommend shorter spells of night work and more time to recover between the shifts (1,2)

  • The number of consecutive night shifts should be low, preferably the maximum of 3
  • Quick returns (<11 hours) should be avoided
  • The use of quickly rotating shift systems (e.g. 2–3 consecutive night shifts) is recommended compared to the use of more slowly rotating schedules (4 or more consecutive night shifts)
  • Pregnant women should not have more than one night shift in a week

The WOW Project “Working hours, health, well-being and participation in working life (WOW) – creating new working time models and solutions for Nordic countries has developed evidence-based models and solutions related to working hours in order to support health, well-being and work participation. The researchers from Finland, Denmark, Norway and Sweden have already produced over 100 scientific publications. The project is financed by NordForsk. The WOW Working Time Policy Symposium is streamed on 13th November to present WOW research results and the recommendations.

Additional information:

Mikko Härmä, Research Professor, Finnish Institute of Occupational Health (FIOH), tel. +35840 544 2750, email mikko.harma[at]

Anne Helene Garde, Professor, National Research Centre for the Working Environment, Denmark, +45 3916 5258; ahg [at] (ahg[at]nfa[dot]dk)

Dr. Johnni Hansen, Danish Cancer Society Research Center, +45 29259193/+45 35257500, johnni [at] (johnni[at]cancer[dot]dk)

WOW Report: Working hours, health, wellbeing and participation in working life. Current knowledge and recommendations for health and Safety
WOW Project website
WOW Recommendations



1. Härmä M and Karhula K (editors). Working hours, health, well-being and participation in working life. Current knowledge and recommendations for health and safety. Finnish Institute of Occupational Health, 2020.

  1. Garde AH, Begtrup L, Bjorvatn B, Bonde JP, Hansen J, Hansen Å M, et al. How to schedule night shift work in order to reduce health and safety risks. Scand J Work Environ Health. 2020;46(6): 
  2. IARC. Night shift work. IARC Monogr Identif Carcinog Hazards Hum: IARC; 2020. p. 1-371. 
  3. Begtrup LM, Specht IO, Hammer PEC, Flachs EM, Garde AH, Hansen J, et al. Night work and miscarriage: a Danish nationwide register-based cohort study. Occup Environ Med. 2019;76(5):302-8. 
  4. Nielsen HB, Dyreborg J, Hansen ÅM, Hansen J, Kolstad HA, Larsen AD, et al. Shift work and risk of occupational, transport and leisure-time injury. A register-based case-crossover study of Danish hospital workers. Safety Science. 2019;120:728-34. 
  5. Nielsen HB, Hansen Å M, Conway SH, Dyreborg J, Hansen J, Kolstad HA, et al. Short time between shifts and risk of injury among Danish hospital workers: a register-based cohort study. Scand J Work Environ Health. 2019;45(2):166-73. 
  6. Vedaa O, Harris A, Erevik EK, Waage S, Bjorvatn B, Sivertsen B, et al. Short rest between shifts (quick returns) and night work is associated with work-related accidents. Int Arch Occup Environ Health. 2019;92(6):829-35. 
  7. Härmä M, Koskinen A, Sallinen M, Kubo T, Ropponen A, Lombardi DA. Characteristics of working hours and the risk of occupational injuries among hospital employees: a case-crossover study. Scand J Work Environ Health. 2020;46(6):570-578.  
  8. Ropponen A, Koskinen A, Puttonen S, Härmä M. Exposure to working-hour characteristics and short sickness absence in hospital workers: A case-crossover study using objective data. Int J Nurs Stud. 2019;91:14-21. 
  9. Larsen AD, Ropponen A, Hansen J, Hansen ÅM, Kolstad HA, Koskinen A, et al. Working time characteristics and long-term sickness absence: a large register-based study of Danish and Finnish nurses. International Journal of Nursing Studies. 2020:103639.

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