Taking nap for more than an hour during the day time may be a warning sign for type-2 diabetes, says a new study.
A group of Japanese researchers have found this link after analysing an observational study that involved more than 3,00,000 people, reports the BBC.
Also known as hyperglycemia, type 2 diabetes is the most common form of insulin resistance diabetes.
UK experts said that people with long-term illnesses and undiagnosed diabetes often felt tired during the day. But they said there was no evidence that napping caused or increased the risk of diabetes.
The study, carried out by scientists at the University of Tokyo, found that there was a link between long daytime naps of more than 60 minutes and a 45 percent increased risk of type-2 diabetes, compared with no daytime napping, but there was no link with naps of less than 40 minutes.
According to the researchers, long naps could be a result of disturbed sleep at night, potentially caused by sleep apnoea. This could even increase the risk of heart attacks, stroke, cardiovascular problems and other metabolic disorders, including type-2 diabetes.
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The risk of type-2 diabetes can even increase due to sleep deprivation, caused by work or social life patterns.
But 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.
Shorter naps, in contrast, were more likely to increase alertness and motor skills, the authors said.
Naveed Sattar, professor of metabolic medicine at the University of Glasgow, 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."
However, he even mentioned that proper trials were needed to determine whether sleeping patterns made a difference to "real health outcomes".
Dr Benjamin Cairns, from the cancer epidemiology unit at the University of Oxford, said the findings should be treated with caution, "In general, it is not possible to make conclusions about cause and effect based on observational studies alone, because usually they cannot rule out alternative explanations for their findings."
The study is being presented at a meeting of the European Association for the Study of Diabetes in Munich.