They searched Medline, the Cochrane Library, and Web of Science for articles published up to November 2014 using the keywords daytime sleepiness, nap, and diabetes. Among 683 studies initially identified, a total of 10 were deemed of good quality and included 261,365 Asian and Western subjects.
The studies came from Sweden, Spain, Finland, and Germany (daytime sleepiness) and the US, China and Germany (napping).
Excessive daytime sleepiness was defined as answering yes to questions like "Do you have a problem with sleepiness during the daytime?"
Daytime napping was defined on the basis of answering yes to questions such as "Do you take a daytime nap?" or "Do you sleep during the day?"
Excessive daytime sleepiness was found to increase the risk of diabetes by 56 per cent, while a longer daytime nap of 60 minutes or more increased the risk by 46 per cent.
In contrast, a shorter nap (60 mins or less per day) did not increase the risk of diabetes. The analysis showed there was no effect of napping up to about 40 minutes per day, after which risk began to increase sharply.
"Excessive daytime sleepiness and taking longer naps were associated with increased risk of type 2 diabetes, with a short nap not increasing this risk," the authors concluded.
"Daytime napping might be a consequence of night-time sleep disturbance such as obstructive sleep aponea (OSA)," they said.
"Epidemiological studies have shown that obstructive sleep apnoea is independently linked to blockages (ischaemia) of heart arteries, stroke, fatal and non-fatal cardiovascular events, and all-cause mortality," they said.
"Several studies have demonstrated the beneficial effects of taking short naps less than 30 minutes in duration, which help to increase alertness and motor skills.
"A short nap finishes before the onset of deep slow-wave sleep. Entering deep slow-wave sleep and then failing to complete the normal sleep cycle can result in a phenomenon known as sleep inertia, in which a person feels groggy, disoriented, and even sleepier than before napping," the researchers said.
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