Night owls at increased risk for higher BMI, larger waistlines, and type 2 diabetes
Individuals who stay up late and wake up later, often referred to as “night owls,” are more likely to have higher body mass index (BMI), larger waist circumferences, and more hidden body fat. Furthermore, they face an almost 50% greater likelihood of developing type 2 diabetes (T2D) compared to those who follow earlier sleep schedules. These findings are part of new research set to be presented at the Annual Meeting of the European Association for the Study of Diabetes (EASD) in Madrid, Spain (9–13 September, 2024).
Lead researcher Dr Jeroen van der Velde, from Leiden University Medical Centre in the Netherlands, explained: “Previous studies have shown that a late chronotype – a preference for going to bed late and waking up late – is linked to an unhealthy lifestyle. Night owls are more prone to smoking or maintaining unhealthy diets, which has been suggested as a possible reason for their higher risk of obesity and metabolic conditions, such as type 2 diabetes.”
Dr van der Velde continued, “However, we believe that lifestyle factors alone cannot fully account for the relationship between being a late chronotype and the development of metabolic disorders. While it is well-documented that late chronotypes tend to have higher BMI, it remains unclear how exactly sleep timing affects the distribution of body fat.”
To investigate this further, Dr van der Velde and his team explored the associations between sleep timing, type 2 diabetes, and body fat distribution in more than 5,000 individuals as part of the Netherlands Epidemiology of Obesity study, an ongoing research project focused on the impact of body fat on health outcomes.
The analysis involved participants with an average age of 56 years, 54% of whom were women, and a mean BMI of 30 kg/m², placing many in the category of obesity. Participants completed a questionnaire about their typical bedtimes and wake-up times, from which researchers calculated their midpoint of sleep (MPS).
The participants were categorised into three groups:
- Early chronotype (the 20% with the earliest MPS),
- Late chronotype (the 20% with the latest MPS),
- Intermediate chronotype (the remaining 60% of participants).
Researchers measured BMI and waist circumference in all participants. Additionally, visceral fat and liver fat content were evaluated in 1,526 individuals using advanced imaging techniques such as MRI scans and MR spectroscopy.
Over a follow-up period of 6.6 years, 225 participants were diagnosed with type 2 diabetes. The data were adjusted for multiple factors, including age, sex, education, total body fat, and various lifestyle habits, such as physical activity levels, diet quality, alcohol consumption, smoking status, and sleep quality and duration. The findings revealed that those identified as late chronotypes had a 46% higher risk of developing type 2 diabetes compared to individuals with an intermediate chronotype.
This significant increase in risk suggests that lifestyle factors alone cannot explain the heightened risk of type 2 diabetes in late chronotypes.
“We believe that other mechanisms are contributing to this increased risk,” said Dr van der Velde. “A likely explanation is that late chronotypes experience misalignment between their natural circadian rhythms and the work and social schedules imposed by society. This misalignment can disrupt their circadian system, which is known to result in metabolic disturbances, potentially leading to type 2 diabetes.”
The researchers also investigated whether early chronotypes – those who prefer to go to bed and wake up earlier – were at a lower risk of developing type 2 diabetes. “From the literature, we expected early chronotypes to exhibit a risk level similar to those with intermediate chronotypes,” Dr van der Velde said. “Our results did show a slightly increased risk for early chronotypes, but this finding was not statistically significant.”
Beyond the risk of diabetes, the results also highlighted key differences in body composition. Late chronotypes, on average, had a BMI that was 0.7 kg/m² higher than those with an intermediate chronotype. They also had a 1.9 cm larger waist circumference, 7 cm² more visceral fat, and 14% greater liver fat content.
Dr van der Velde concluded, “Individuals with a late chronotype seem to be at greater risk for type 2 diabetes compared to those with an intermediate chronotype. This elevated risk might be due to higher levels of body fat, including visceral fat and liver fat.”
Looking ahead, Dr van der Velde emphasised that the next steps in research will involve studying whether individuals with late chronotypes can improve their metabolic health by adjusting the timing of their lifestyle behaviours. He mentioned that they are currently involved in the TIMED consortium, which investigates how the timing of sleep, eating, and physical activity can influence type 2 diabetes outcomes.
“We’ve already demonstrated that the timing of physical activity plays an important role in reducing insulin resistance,” he noted.
Another promising area of research is exploring the impact of meal timing. “It’s likely that people with late chronotypes are more inclined to eat later into the evening,” explained Dr van der Velde. “While we didn’t examine this in our study, there is growing evidence that time-restricted eating – such as not consuming food after a certain hour, like 6pm – could lead to metabolic benefits.”
For individuals who identify as night owls and are concerned about their increased risk for type 2 diabetes, Dr van der Velde suggested that they may want to experiment with adjusting meal times or avoiding late-night eating. “The evidence isn’t definitive yet, but over time, we aim to provide more specific advice on how changes in lifestyle habits can reduce the risk of metabolic diseases.”