In a new review of previously published studies, people who took daytime naps longer than an hour were 45 percent more likely to have type 2 diabetes compared with those who didn’t nap at all. The association does not show that napping leads to diabetes, but does suggest that it could be a warning sign of the disease.
People who regularly napped for less than 60 minutes at a time had no increased risk.
The new review, conducted by researchers at the University of Tokyo, was presented at this week’s European Association for the Study of Disease annual meeting in Munich; it has not yet been peer-reviewed or published in an academic journal.
Napping is prevalent around the world, said study author Yamada Tomahide in a press release. It’s especially popular with people who can’t get enough sleep at night, for social or work-related reasons. Naps can also be helpful for people with sleep disorders who suffer from excessive daytime sleepiness.
And while getting enough sleep is important for overall wellness, too much sleep can also be considered a risk factor for (or a symptom of) chronic health problems. In his research, Tomahide cited several recent studies that have shown U-shaped curves describing the relationship between hours of sleep a night with metabolic diseases.
For this review, Tomahide and his colleagues looked at 21 studies with a total of 307,237 participants from Asian and Western countries. In each study, they note, researchers had designed their analyses to rule out possible influencers such as age, gender, and underlying health conditions.
Together, the results from these studies formed a J-shaped curve describing the relationship between daily nap duration and the risk of diabetes or metabolic syndrome. (Metabolic syndrome is a cluster of symptoms, including high blood pressure, obesity, and high cholesterol, that is often a precursor to full-blown diabetes or heart disease.)
The news isn’t all bad: Naps of any duration were not associated with an increased risk of obesity. And short naps actually appeared to be associated with a slightly lower risks or diabetes and metabolic syndrome, compared to no naps at all, although those results were not clinically significant.
As naps got longer, however, the risks of having diabetes and metabolic syndrome began to rise sharply, becoming clinically significant at 60 minutes and up.
This does not mean that napping itself raises the risk for diabetes, say the study authors. Rather, diabetes and the need for daytime naps could potentially share a common cause.
For example, someone who takes long naps every day could be doing so because they have a nighttime sleep disturbance like obstructive sleep apnea—a chronic condition linked to several other serious health problems.
Napping could also indicate sleep deprivation for other reasons. Regardless of cause, sleep deprivation has been shown to increase hunger and have a harmful impact on hormones and metabolism, potentially raising a person’s risk for diabetes. Depression, another reason people tend to sleep more, is also associated with diabetes.
Eve Van Cauter, PhD, an expert on circadian rhythm and metabolism at the University of Chicago, agrees that people who take long naps are likely to have other risk factors for diabetes.
“If they’re that tired during they day, it is likely that they are either having insufficient sleep or poor-quality sleep at night,” she says, “or they have an underlying condition, like depression, that the study did not look at.” She also singles out obstructive sleep apnea as a common cause of poor-quality sleep and daytime sleepiness, and says that it is “associated with strong risk factors for prediabetes and diabetes.”
Van Cauter, who was not involved in this new study, said she’d also be interested to know if shift workers or people with irregular work schedules were included in Tomahide’s analysis. “Shift work affects 20 percent of the active population and is also a risk factor for diabetes,” she says.
A short nap could be a smarter choice than a longer one, the researchers say, because short naps don’t involve deep-wave sleep. When people enter deep-wave sleep but don’t complete a full sleep cycle, they can experience sleep inertia—a feeling of grogginess, disorientation, and greater sleepiness than before.
“Several studies have demonstrated beneficial effects of taking short naps less than 30 minutes in duration, which help to increase alertness and motor skills,” they said. “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 finding.”
In other words, they say, short naps may help improve circadian rhythm problems or endocrine abnormalities caused by sleep deprivation, while longer naps may not.
More studies are needed to determine how and why daytime sleeping—of any duration—truly affects diabetes risk. For now, if you’re sleeping for longer than an hour every afternoon, it might be a good idea to ask yourself (or your doctor) if an underlying health condition might be playing a role.
This article originally appeared on RealSimple.com.