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Night shift work increases the risk for type 2 diabetes
  1. John Axelsson1,
  2. Sampsa Puttonen2
  1. 1Clinical Neuroscience, Division of Psychology, Karolinska Institute, Stockholm, Sweden
  2. 2Finnish Institute of Occupational Health, Helsinki, Finland
  1. Correspondence to John Axelsson
    Karolinska Institute, Department. of Clinical Neuroscience, Division of Psychology, 17177 Stockholm, Sweden; john.axelsson{at}

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As a result of the rapidly evolving 24-h society, about 15–30% of the workforce works outside normal business hours, with about half of them working night shifts. Any shift worker knows that night work compromises cognitive capacity and challenges the physiological need for sleep and recuperation. It is not surprising therefore that night shift work relates to a variety of health problems, including sleep disturbances, gastro-intestinal disorders, cancer and a number of metabolic disorders such as obesity, the metabolic syndrome, cardiovascular disease and type 2 diabetes.1 However, many previous studies have not fully captured the relationship between these health problems and shift work exposure.


Over 177 000 women from two large prospective cohort studies (Nurses' Health Study I and II) covering ages 25–67 years were followed up for 20 years. They rated whether they had worked at least 3 night shifts per month in addition to days and evenings in that month. They were re-surveyed at 2–4 year intervals. The effect of shift work on the self-reported incidence type 2 diabetes, measured 18–20 years later, was tested using Cox proportional hazards regression models with adjustment for relevant confounders.


Both cohorts showed a gradual increase in type 2 diabetes risk with increased exposure to shift work. Compared with day workers, the pooled HRs for having worked 1–2, 3–9, 10–19, ≥20 years of shift work were 1.05, 1.20, 1.40 and 1.58, respectively. Exposure to shift work was also related to weight gain, and adjustments for body mass reduced the associations with diabetes risk by about 50%.


The study by Pan et al demonstrates a clear association between shift work and type 2 diabetes. This is in line with previous studies reporting an association between shift work and several metabolic disorders, including type 2 diabetes.1 Strengths of the study include a large sample size, long follow-up period and good-quality data on shift work exposure. Type 2 diabetes status was self-reported and was further confirmed with supplementary questionnaires. Type 2 diabetes can probably be reliably measured with self-reports in nurses, even if it may be insufficient in other groups. Importantly, this study supports that shift work is also a metabolic risk factor for younger workers, and not just a problem with increasing age. Correspondingly, young shift workers also have an increased risk to develop subclinical atherosclerosis.2

The results confirm and extend the findings of a Swedish study reporting an increased risk for diabetes-related mortality with exposure to shift work.3 Potential methodological problems of both these studies concern how to properly analyse exposure and the ‘healthy worker effect’. Self-ratings of exposure to shift work may not be fully reliable since it is difficult to adequately sum up whether one has worked three nights in a month in a varying shift schedule. An example of the ‘healthy worker effect’ is that shift workers with a high risk of developing diabetes are twice as likely to leave their working organisations.4 Thus, the risks of exposure to night shift work are probably underestimated. To sum up, the results showed that working ≥3 night shifts per month increases the risk of type 2 diabetes. In order to generate guidelines for exposure and ergonomic shift schedules, additional knowledge is needed on how many nights a person can work safely in a month.

Several strategies can reduce health risks among shift workers. First, organisations should acknowledge that shift work is a metabolic risk factor and actively use strategies to improve life-style. Second, health screening for individuals who may be vulnerable to shift work or its health effects could be conducted during recruitment of employees.5 Third, regular health check-ups are needed in young shift workers. Even a simple screening questionnaire can efficiently detect the risk for metabolic disorders.6 Fourth, use of ergonomic working schedules and individual solutions should be adopted more widely. For example, changing to a more ergonomic schedule improves blood lipids and blood pressure. Lastly, shift-workers need to be educated to cope better with shift-work (eating, sleeping and exercising) and the use of countermeasures (eg, naps) should be widened.


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  • Competing interests None.