Those who nap for more than an hour a day have a 45 per cent increased risk of having type 2 diabetes, researchers found.
But shorter naps did not show an increased risk, according to the study which is to be presented to the European Association for the Study of Diabetes (EASD) meeting in Munich, Germany.
Experts from Japan looked at data from 21 studies concerning more than 300,000 people and found the association between daytime snoozes and type 2 diabetes.
After adjusting for potential factors, they found that a long nap of more than 60 minutes a day "significantly increased the risk of type 2 diabetes". But naps of less than an hour did not.
"Longer nap was associated with increased risk of diabetes and metabolic syndrome," the authors concluded.
They said that short naps might have beneficial effects on diabetes, but added: "Further studies are needed to confirm the efficacy of a short nap."
They point out that sleep is an important part of a healthy life alongside a good diet and exercise.
"Some people cannot get enough sleep at night due to social life and work life related factors," they wrote.
"Taking naps is widely prevalent around the world.
"Daytime naps are usually brief, but can range from a few minutes to a few hours.
"The frequency varies from taking an occasional nap to planned rest periods even several times daily for habitual nappers.
"Some individuals take a nap because they are excessively sleepy during the daytime as a result of a sleep disorder."
