Good sleep is important for reducing the long-term risk of chronic conditions like diabetes and obesity. But is it just the total hours of sleep that matter, or does the timing of when you go to bed play a role? Researchers now suggest that night owls, the sleep chronotype that prefers staying up late and waking up later, are at a higher risk of developing type 2 diabetes.

Although night owls have long been linked to unhealthy habits like poor diet and smoking that typically elevate the risk of metabolic conditions, recent research reveals that the increased diabetes risk in them might not be just about lifestyle choices.

According to the latest study, which will be presented at the Annual Meeting of the European Association for the Study of Diabetes (EASD) in Madrid, Spain, fat distribution also contributes to night owls' increased diabetes risk. Night owls tend to have larger waistlines and more hidden body fat, making them nearly 50% more likely to develop type 2 diabetes.

"We believe that lifestyle cannot fully explain the relationship between a late chronotype and metabolic disorders. In addition, while it is known that a late chronotype is associated with high BMI, it isn't clear to what extent chronotype affects body fat distribution," said the lead researcher Dr Jeroen van der Velde, of Leiden University Medical Centre, Leiden, Netherlands in a news release.

To investigate the link between sleep timing, type 2 diabetes, and body fat distribution, the researchers evaluated 5,000 participants in the Netherlands Epidemiology of Obesity study, which examines the influence of body fat on disease.

The study involved participants with an average age of 56 and a mean BMI of 30 kg/m². Researchers used questionnaires to record their typical bed and wake times, determining the midpoint of their sleep. Participants were then grouped into three: early chronotype (the 20% with the earliest sleep midpoint), late chronotype (the 20% with the latest sleep midpoint), and intermediate chronotype (the remaining 60%).

The team also measured all participants' BMI and waist circumference. Their visceral fat and liver fat were measured using MRI scans and MR spectroscopy, respectively. After a median follow-up of 6.6 years, 225 participants were diagnosed with type 2 diabetes.

Compared with an intermediate chronotype, participants with a late chronotype had a 46% higher risk of type 2 diabetes, even after adjusting for other factors including age and lifestyle factors, researchers noted. This indicates that lifestyle factors alone cannot explain an increased risk of type 2 diabetes in late chronotypes.

"We believe that other mechanisms are also at play. A likely explanation is that the circadian rhythm or body clock in late chronotypes is out of sync with the work and social schedules followed by society. This can lead to circadian misalignment, which we know can lead to metabolic disturbances and ultimately type 2 diabetes," explained Dr van der Velde.

"People with a late chronotype appear to be at greater risk of developing type 2 diabetes compared to those with intermediate chronotype, possibly because of higher body fat including more visceral fat and liver fat. The next step is to study if those with a late chronotype improve in metabolic health when they make changes in the timing of their lifestyle habits," Dr van der Velde concluded.

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