Daytime drowsiness and taking long naps during the day was associated with an increased prevalence of Type 2 diabetes, according to a review of past studies.

Researchers at the University of Tokyo reviewed 10 studies including a total of more than 260,000 individuals, looking for links between daytime sleepiness or napping and the risk of Type 2 diabetes.

The doctors found that those who felt very sleepy during the day had a 56% higher risk of being diabetic than those who didn’t, while those who took naps of 60 minutes or more had a 46% higher risk of having the condition than those who took no or smaller naps.

The study, to be presented at the European Association for the Study of Diabetes conference in Stockholm on Friday, shows an association between the two conditions but doesn’t prove cause and effect. Still, researchers say their findings suggest excessive sleepiness and long napping during the day may be a warning signal of Type 2 diabetes.

People who experience these conditions should see a doctor to have their symptoms evaluated, said Dr. Tomohide Yamada, a researcher at the department of diabetes and metabolic diseases at the University of Tokyo in Japan.

Dr. Yamada noted the possibility of reverse causality, meaning people who are already suffering from bad health may be more inclined to feel sleepy or take longer daytime naps.

The analysis showed there was no adverse link between short naps of about 40 minutes a day and Type 2 diabetes. Previous studies have found beneficial health effects of taking naps of 30 minutes or less a day including increased alertness and improved motor skills.

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