Researchers at the University of Tokyo say that napping for more than an hour during the day could be an indicator of type-2 diabetes.
The study, which involved data from 21 studies and over 300,000 people, found a non-causal link between daytime naps of more than 60 minutes and a 45 per cent increased risk of type-2 diabetes. There was no link between diabetes and naps of less than 40 minutes.
The research is being presented at a meeting of the European Association for the Study of Diabetes in Munich.
The study’s authors say that shorter naps are likely to increase alertness and motor skills, but there is no consensus on the best length of nap time. A 1995 study by Nasa concluded that 26 minutes is ideal. At this length of time, performance and alertness in air traffic controllers was found to increase, by 34 per cent and 54 per cent respectively.
The study’s lead author, Dr Yamada Tomahide, said: ‘Several studies have demonstrated 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.
‘Although the mechanisms by which a short nap might decrease the risk of diabetes are still unclear, such duration-dependent differences in the effects of sleep might partly explain our finding. A short nap might have the effect of improving an abnormal circadian rhythm and modifying a variety of endocrine abnormalities caused by sleep deprivation.’
What we know so far is that sleep disturbances per se are associated with insulin resistance. Insulin resistance — that is, when cells fail to respond properly to insulin — is often the precursor to type-2 diabetes.
As the paper has not yet been published, it is difficult to know what variables the researchers took into account. Napping as a result of sleep apnoea at night, for example, is a red herring, as it is the sleep disturbance at night which is the problem.
On the other hand, extreme sloth, whether related to sheer laziness, or as a result of eating junk food that renders the consumer semi-comatose, will of course be associated with napping, which will then be associated with diabetes.
Napping can simply be a result of extreme fatigue. This may be caused by all manner of diseases associated with diabetes, for instance Cushing’s disease.
The problem with population-based case-control or cohort studies is that associations will always be found and it can often be forgotten that they are merely that: associations and not causations. Napping might be associated with diabetes, but it most certainly does not cause it.
So this is an interesting study, but no one should be worried about napping itself — it has, among its other benefits, been shown to enhance memory.
I would suggest, however, that anyone finding that they are excessively tired during the day regardless of how much they sleep be tested for thyroid disease, possible adrenal issues, diabetes, sleep apnoea or any other condition your GP feels is warranted, before putting the problem down to stress.
Research score: N/A