Sleepiness during the day and taking long daytime naps shows an association with an increased prevalence of type 2 diabetes, say Japanese researchers after reviewing previously-published studies.

An analysis of 10 studies involving more than 260,000 people revealed a link between sleepiness and napping and an elevated risk of developing the condition, researchers at the University of Tokyo are reporting.

There was a 56 percent higher risk of diabetes for people who reported feeling very sleepy during the day compared with those who didn't, while there was a 46 percent higher risk for those who took naps of an hour or more during the day compared with those who took shorter naps or no naps at all, the researchers found.

The study being presented at the European Association for the Study of Diabetes conference in Stockholm has found a link or an association, but that is not the same as proving a cause and effect, the researchers are careful to point out.

Even if that were so, the causality could be operating in reverse, says Dr. Tomohide Yamada of the university's department of diabetes and metabolic diseases.

It may be the case that people already suffering from poor health with conditions such as diabetes may be more likely to feel sleepy during the day and take longer naps, he explains.

In addition, the researchers note, there are other conditions such as obstructive sleep apnea or nighttime sleep disturbances that can leave people feeling sleepy during the day and wanting to nap.

People with depression are also likely to be prone to episodes of sleepiness and napping, they say.

Additionally, both sleep disturbances and depression have been linked to increased risk of diabetes in previous research.

Still, Yamada says, the findings suggest napping and sleepiness could be considered warning signals of possibly developing type 2 diabetes.

People who are consistently feeling sleepy or needing naps without any apparent cause should consider an exam by a doctor, he suggests.

Short naps — 40 minutes or less — did not show any link with the condition, the researchers found, and many studies have indicated significant health benefits, such as improved motor skills and mental alertness, among people taking shorter naps in the daytime.

One of the most prevalent diseases worldwide, diabetes affects more than 380 million people worldwide.

It can result in blindness, kidney failure, heart disease and even death.