Previous studies have demonstrated the beneficial effects of taking short naps, but the new analysis suggests long naps could increase the risk of type 2 diabetes.
The research is presented at the 51st Annual Meeting of the European Association for the Study of Diabetes, taking place in Stockholm, Sweden.
Getting enough sleep is a key part of a healthy lifestyle, playing a crucial role in protecting both physical and mental health. Not getting enough sleep can lead to sleepiness during the day along with napping - the habit of taking short sleeps ranging from a few minutes to a few hours.
According to the National Sleep Foundation, a short nap of 20-30 minutes can help improve mood, alertness and performance. However, napping for longer than 10-20 minutes can lead to sleep inertia - a feeling of disorientation that comes from waking after a deep sleep - as well as potentially having a negative effect on the length and quality of nighttime sleep.
In the new study, Dr. Tomohide Yamada, of the University of Tokyo, Japan, and colleagues set out to examine the association between daytime sleepiness, napping and the risk of type 2 diabetes by conducting a meta-analysis of research published up to November 2014.
Many of the risk factors for type 2 diabetes are linked to lifestyle, including physical inactivity, excess weight and poor diet.
Excessive sleepiness linked to 56% increase in type 2 diabetes risk
- In the US, 29.1 million people have diabetes, though 27.8% of these cases are undiagnosed
- Around 90% of people with diabetes are estimated to have type 2 diabetes
- The risk of type 2 diabetes increases with age.
The researchers identified 10 studies that were suitable for the meta-analysis, involving a total of 261,365 subjects. These studies utilized self-reporting to determine daytime sleepiness and napping, with questions such as "Do you have a problem with sleepiness during the daytime?" to measure the participants' sleep habits.
Risk of type 2 diabetes increased by 56% among those who reported excessive daytime sleepiness. While napping for longer than 60 minutes during the day increased the risk of type 2 diabetes by 46%, naps that were shorter than this did not affect diabetes risk.
"Excessive daytime sleepiness and taking longer naps were associated with increased risk of type 2 diabetes, with a short nap not increasing this risk," the authors conclude.
Regarding these findings, the authors suggest that mechanisms behind short naps - demonstrated in several studies to have beneficial effects - could explain things, stating that short naps finish before the onset of deep slow-wave sleep:
"Entering deep slow-wave sleep and then failing to complete the normal sleep cycle can result in a phenomenon known as sleep inertia, in which a person feels groggy, disoriented, and even sleepier than before napping. Although the mechanisms by which a short nap might decrease the risk of diabetes are still unclear, such duration-dependent differences in the effects of sleep might partly explain our findings."
The researchers also point out that daytime napping could be caused by nighttime sleep disturbance such as obstructive sleep apnea. This condition is associated with ischemia, stroke, cardiovascular events and all-cause mortality, and also shares some risk factors with type 2 diabetes, such as excess weight and age.
Earlier this year, Medical News Today reported on a study that found people who work shifts are more likely to have sleep problems than people who follow conventional work schedules, in turn increasing their risk of metabolic disorders such as diabetes and obesity.