People who slept more than an hour each day were 45 per cent more likely to develop type 2 diabetes, a debilitating condition associated with overweight and a sedentary lifestyle, the study found.
Individuals with diabetes are unable to naturally regulate their blood sugar levels.
Without treatment, the disease can lead to blindness, nerve damage, kidney failure, heart disease and premature death.
The meta-analysis – a review of existing scientific literature – by Yamada Tomahide of the University of Tokyo and colleagues, covered more than 300,000 people of Asian and European origin from around the world.
For naps of less than forty minutes, the link with diabetes disappeared, they noted.
Experts not involved in the research noted that the statistical correlation revealed nothing about cause-and-effect.
The study, they added, has yet to be published in a peer-reviewed science journal.
“It can falsely appear that their illness followed increased napping, rather than the other way around,” said Benjamin Cairns, a researcher at the Cancer Epidemiology Unit of the University of Oxford.
“This could mean that long naps appear to cause diabetes or other diseases, even when only the reverse is true.”
Naveed Sattar, an expert on metabolic disease at the University of Glasgow said the results should be taken with a generous pinch of salt.
“It’s likely that risk factors which lead to diabetes also cause napping,” he wrote in a comment distributed by the non-profit Science Media Centre.
Only clinical trials – experiments in which scientists could test the impact of napping by comparing two or more groups of people – can reveal whether too much day-time sleeping is bad for health.
“Without proper trials, we simply will never know the answer,” Sattar said.
The study was unveiled at a meeting of the European Association for the Study of Diabetes in Munich, Germany.