People with a common breathing disorder which strikes while they are asleep could be at increased risk of getting diabetes later in life.
People with apnoea may have hidden diabetes
Scientists have found a link between obstructive sleep apnoea and early signs of diabetes.
They say it is possible the side-effects of apnoea could be causing permanent damage to their metabolism.
Sleep apnoea leads an interruption of breathing during sleep - and affects one in ten middle-aged men.
Being obese makes the condition much more likely.
Sleep apnoea is potentially damaging because it not only reduces the oxygen concentration in the blood stream, but also disturbs sleep patterns and causes daytime fatigue.
This relationship cannot be explained by the known risk factors
Dr Nicole Meslier, Angers Hospital
Researchers from Angers University of Hospital in France noticed the group of men diagnosed with sleep apnoea were also those apparently more prone to early signs of diabetes.
They wanted to find out whether there was a connection between the two.
Almost 700 men with suspected sleep apnoea also had blood tests before and after a "meal" of glucose to see how well their body dealt with the sugar.
They were surprised by the findings - half of the apnoea patients seemed to have signs of a diabetes-related metabolic disorder.
Almost a third of those with confirmed apnoea could actually be defined as diabetic - and two-fifths of these cases were previously undiagnosed.
Dr Nicole Meslier, one of the researchers, said: "The degree of insulin resistance correlated with the severity of the sleep apnoea.
"This relationship cannot be explained by the known risk factors, such as age and weight, which we factored into our statistical analysis, even though our apneic patients were, on average, heavier than the ordinary snorers."
The precise reasons for the connection cannot be proven, although there is some evidence that the oxygen deprivation caused by sleep apnoea might be harmful.
Treatments are available for the worst sufferers, who are given supplemental oxygen through a mask at night to compensate for their condition.
However, the researchers, writing the European Respiratory Journal, strongly recommend anyone with sleep apnoea gets tested for diabetes.