Napping for more than an hour during the day could be a warning sign for type-2 diabetes, new research suggests.

The study, carried out by the University of Tokyo, analysed observational studies of more than 300,000 people people of Asian and European origin from around the world.

The research highlighted a link between long daytime naps that were more than 60 minutes long and a 45% increased risk of type-2 diabetes when compared with no daytime napping. However, there was no link with naps of less than 40 minutes.

The study was presented today at a meeting of the European Association for the Study of Diabetes in Munich.

The study's lead author, Dr Yamada Tomahide, stressed that it was an observational study and the results don’t prove that napping causes Type 2 diabetes.

He told the European Association for the Study of Diabetes meeting, that sleep was a key component of a healthy life along with a good diet and exercise.

Dr Tomahide said: “A longer nap was associated with increased risk of diabetes and metabolic syndrome. Further studies are needed to confirm the efficacy of a short nap.”

Picture by: Peter Byrne / PA Archive/Press Association Images

Individuals with diabetes are unable to naturally regulate their blood sugar levels. If left untreated the disease can lead to blindness, nerve damage, kidney failure, heart disease and premature death.

Prof Naveed Sattar, Professor of Metabolic Medicine at the University of Glasgow, said:

“This observational study shows an association between longer naps and diabetes. 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.

“That said, there is now abundant evidence of a link of sorts between sleep disturbances and diabetes. What we need are trials to determine if, when and for how long one sleeps makes a difference to real health outcomes.

"Trials bring truth and without proper trials, we simply will never know the answer.”