A study of more than 60,000 middle-aged nurses found that people with an evening chronotype, or a circadian preference to feel energetic later in the day, experienced an increased risk for diabetes and were more likely to report unhealthy lifestyle behaviors, such as smoking, poor sleep, and physical inactivity, compared with persons with a morning chronotype. 

The findings are published in Annals of Internal Medicine.

An estimated 8% of the population has an evening chronotype, which has been linked to poor metabolic regulation, disrupted glycemic control, metabolic disorders, and higher incidence and prevalence of type 2 diabetes. However, the reasons for the observed association between evening chronotype and increased diabetes risk are not well understood.

Researchers from Brigham and Women’s Hospital and Harvard Medical School conducted a prospective cohort study of 63,676 nurses aged 45 to 62 years with no history of cancer, cardiovascular disease, or diabetes from 2009 to 2017. The authors found that participants with a “definite evening” chronotype were 54% more likely to have an unhealthy lifestyle than participants reporting a “definite morning” chronotype. Persons with evening chronotype also had a 72% higher risk of developing diabetes during the follow-up period. 

The authors report that this association weakened but persisted even after adjusting for all measured lifestyle and sociodemographic factors. They emphasize that these results are restricted to persons who did not work recent night shifts. Future investigation in other populations leveraging genetic determinants for chronotype is needed to determine whether their findings are applicable to men, non-White racial or ethnic groups, or other socioeconomic classes. Moreover, generational differences in diet, exercise, and body weight may limit the applicability of their findings to younger or older generations or current times.

An accompanying editorial by authors from the Harvard T.H. Chan School of Public Health and Harvard Medical School highlights that several factors, including psychological factors, type of work, and possible lifetime changes to chronotype could confound the results of this study. They add that the results suggest that circadian misalignment due to a mismatch between chronotype and work timing, rather than the chronotype, may be a potential mechanism for these results. 

The authors suggest that this study adds to the growing evidence that reassigning evening chronotype workers to night shifts may improve sleep among shift workers and improve their metabolic health. Finally, they note that these results point to the potential benefit of developing standardized tools to assess chronotype regularly throughout a person’s life.

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