With roughly 20 percent of Americans report sleeping less than six hours each night, napping may be beneficial, as it allows people to rejuvenate their brains and energy levels. However, for older adults, there may be some drawbacks to napping. One new study found that older people who napped for longer than 30 minutes had an increased risk for developing type 2 diabetes.

“There is accumulating evidence showing it may also be a risk factor for morbidity and mortality,” the authors said, according to the Daily Mail. But the study’s researchers did not dispute that naps are beneficial to daytime functioning and mental health.

Out of 27,009 Chinese participants ages 45 and older, about 70 percent reported taking naps regularly during the afternoon. Researchers found impaired fasting plasma glucose (IFG), commonly known as prediabetes, increased in a “dose-dependent” manner as naps increased in length. The increase in pre-diabetes was noted only in naps longer than 30 minutes. Someone with blood sugar levels that are higher than normal, but not high enough to be considered diabetes, is considered to have prediabetes.

Despite its name, prediabetes doesn’t necessarily lead to type 2 diabetes, according to the American Diabetes Association. Early treatment and healthy lifestyle changes, such as exercise and eating healthy foods, can return blood sugar to its proper level. Without these lifestyle changes, however, diabetes is likely to develop.

Participants in the study had physical exams, face-to-face interviews, and lab tests to monitor their blood sugar levels. Participants were split into groups based on how long they napped: no napping, less than 30 minutes, 30 to 60 minutes, 60 to 90 minutes, and more than 90 minutes.

The researchers also found that among those who napped, 40 percent had high blood pressure and 24 percent had high cholesterol, compared to 33 percent and 19 percent, respectively, among those who didn't take naps.

The study authors offered two reasons for their findings. For one, taking a daily nap could be cutting into time that could otherwise be used for exercising. Secondly, sleeping during the day could offset the body’s circadian rhythm, exposing internal organs to higher levels of the stress hormone cortisol.

About one-third of Americans report taking naps daily, with older adults — 41 percent of men and 28 percent of women over 50 years old — most likely to nod off, according to Live Science.

Still the study has its skeptics. Dr. Matthew Hobbs, head of research for Diabetes UK, told the Daily Mail that there was no proof that napping actually caused diabetes. “The bottom line is that the best way to reduce your risk for type 2 diabetes is to maintain a health weight by eating a healthy, balanced diet and by being regularly physically active.”

Source: Fang W, Li Z, Cao Z, et al. Longer habitual afternoon napping is associated with a higher risk for impaired fasting plasma glucose and diabetes mellitus in older adults: results from the Dongfeng–Tongji cohort of retired workers. Sleep Medicine. 2013.