In the study led by Yamada Tomahide from the University of Tokyo, the team conducted a meta-analysis of observational studies involving more than 300,000 people.

They found that long daytime naps of more than 60 minutes may lead to a 45 per cent increased risk of Type-2 diabetes, compared with no daytime napping. 

However, it was also possible that people who were less healthy or in the early stages of diabetes were more likely to nap for longer during the day. 

People with long-term illnesses and undiagnosed diabetes often felt tired during the day, the researchers said.

"It's likely that risk factors which lead to diabetes also cause napping. This could include slightly high sugar levels, meaning napping may be an early warning sign of diabetes," Naveed Sattar, Professor at the University of Glasgow, was cited by bbc.com as saying in a comment on the research findings.

There was now a lot of evidence of some kind of link between sleep disturbances and diabetes, Sattar observed.

Long naps could also be a result of disturbed sleep at night, potentially caused by sleep apnea. 

And this sleeping disorder could increase the risk of heart attacks, stroke, cardiovascular problems and other metabolic disorders, including Type-2 diabetes.

Sleep deprivation, caused by work or social life patterns, could also lead to increased appetite, which could increase the risk of Type-2 diabetes.

In contrast, shorter naps (less than 40 minutes), were more likely to increase alertness and motor skills, the researchers noted. 

The findings were presented at 2016 European Association for the Study of Diabetes Meeting in Munich, Germany, recently.