SleepNapping for more than 30 minutes at a time can raise the chances of developing type two diabetes, according to the results of a study of more than 27,000 people in China.

Researchers found men and women taking 40 winks were also more likely to have high blood pressure and raised cholesterol levels compared to those who stayed awake through the day.

The findings, first published in the journal Sleep Medicine, are in contrast to those from other recent studies, which found daytime sleeps could boost brain power and slash the risk of heart attacks and strokes by more than a third.

The researchers said it’s the duration of the nap that counts. Those dozing for half an hour or more were more likely to have the early signs of diabetes than those who snoozed for less time or not at all.

But the remaining 90 per cent are type two, closely linked to unhealthy diet and lifestyle.

The body loses its ability to make use of glucose, a type of sugar that is released when we eat food and turned into a source of energy for use by muscles. As glucose levels rise, circulation starts to suffer and blood vessels in areas such as the heart, the legs and the eyes can be irreparably damaged. In the latest study, researchers at the Huazhong University of Science and Technology in China studied 27,009 men and women aged 45 or over. Almost 70 per cent of the volunteers said they regularly took a nap in the afternoon.

Researchers checked their health by carrying out a test called impaired fasting plasma glucose. This measures whether sugar in the blood is too high and acts as an early warning sign that type two diabetes is setting in. 

Researchers also looked to see which volunteers were in the early stages of the disease. They found glucose readings were much higher among those who favoured a daytime sleep.

Forty per cent of them also had high blood pressure, compared to just 33 per cent of non-nappers, and 24 per cent had high cholesterol, versus 19 per cent.

One reason a siesta may be harmful is it simply means less exercise is being undertaken, the researchers said.

But it could also be that it disrupts the body’s internal clock and exposes organs to higher levels of the stress hormone cortisol.

In a report on their findings the researchers said: ‘Napping in the elderly can be beneficial for daytime functioning, as well as for mental health.

‘But there is accumulating evidence showing it may also be a risk factor for morbidity and mortality.’

Dr Matthew Hobbs, head of research for Diabetes UK, said there was no proof that napping actually caused diabetes.

He said: ‘The bottom line is that the best way to reduce your risk of type two diabetes is to maintain a healthy weight by eating a healthy, balanced diet and by being regularly physically active.