"Night owls" have an increased risk for developing type 2 diabetes and are more likely to smoke more, exercise less, and have poor sleep habits compared with their "early bird" counterparts, according to a new study, published today in the Annals of Internal Medicine.
The work focused on participants' self-assessed chronotype — an individuals' circadian preference, or natural preference to sleep and wake up earlier or later, commonly known as being an early bird or a night owl.
Analyzing the self-reported lifestyle behaviors and sleeping habits of more than 60,000 middle-aged female nurses, researchers from Brigham and Women's Hospital and Harvard Medical School found that those with a preference for waking up later had a 72% higher risk for diabetes and were 54% more likely to have unhealthy lifestyle behaviors compared with participants who tended to wake up earlier.
After adjustment for six lifestyle factors — diet, alcohol use, body mass index (BMI), physical activity, smoking status and sleep duration — the association between diabetes risk and evening chronotype weakened to a 19% higher risk of developing type 2 diabetes.
In a subgroup analysis, this association was stronger among women who either had had no night shifts over the previous 2 years or had worked night shifts for less than 10 years in their careers. For nurses who had worked night shifts recently, the study found no association between evening chronotype and diabetes risk.
The participants, drawn from the Nurses' Health Study II, were between 45 and 62 years age, with no history of cancer, cardiovascular disease, or diabetes. Researchers followed the group from 2009 until 2017.
Is There a Mismatch Between Natural Circadian Rhythm and Work Schedule?
The authors, led by Sina Kianersi, DVM, PhD, of Harvard Medical School, Boston, suggest that their results may be linked to a mismatch between a person's circadian rhythm and their physical and social environment — for example, if someone lives on a schedule opposite to their circadian preference.
In one 2015 study, female nurses who had worked daytime shifts for more than 10 years but had an evening chronotype had the highest diabetes risk compared with early chronotypes (51% more likely to develop type 2 diabetes).
In a 2022 study, an evening chronotype was associated with a 30% elevated risk for type 2 diabetes. The authors speculated that circadian misalignment could be to blame — for example, being a night owl but working early morning — which can disrupt glycemic and lipid metabolism.
Previous studies have found that shorter or irregular sleep habits are associated with a higher risk of type 2 diabetes. Other studies have also found that people with an evening chronotype are more likely than early birds to have unhealthy eating habits, have lower levels of physical activity, and smoke and drink.
This new study did not find that an evening chronotype was associated with unhealthy drinking, which the authors defined as having one or more drinks per day.
In an accompanying editorial, two physicians from the Harvard T.H. Chan School of Public Health in Boston caution that the statistical design of the study limits its ability to establish causation.
"Chronotype could change later, which might correlate with lifestyle changes," write Kehuan Lin, MS, Mingyang Song, MBBS, and Edward Giovannucci, MD. "Experimental trials are required to determine whether chronotype is a marker of unhealthy lifestyle or an independent determinant."
They also suggest that psychological factors and the type of work being performed by the participants could be potential confounders.
The authors of the study note that their findings might not be generalizable to groups other than middle-aged White female nurses. The study population also had a relatively high level of education and were socioeconomically advantaged.
Self-reporting chronotypes with a single question could also result in misclassification and measurement error, the authors acknowledge.
The findings underscore the value of assessing an individuals' chronotype for scheduling shift work — for example assigning night owls to night shifts may improve their metabolic health and sleeping habits, according to the authors of the study.
"Given the importance of lifestyle modification in diabetes prevention, future research is warranted to investigate whether improving lifestyle behaviors could effectively reduce diabetes risk in persons with an evening chronotype," the authors conclude.
The study was supported by grants from the National Institutes of Health and the European Research Council.
Annals of Internal Medicine. Published online September 11, 2023. Abstract
Lori Youmshajekian is a freelance health journalist based in New York.
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