People with bedtime snoring and breathing problems may be using up far more calories while they sleep.
Those with the worst sleep apnoea symptoms burned 373 extra calories a day compared with those with only mild symptoms, US researchers found.
Nervous system changes triggered by the condition may be responsible.
The payback comes in the daytime, said a UK specialist, with sleep apnoea sufferers craving food and too sluggish to exercise.
I used to joke that sleep apnoea was free exercise at night - it's nice to have that confirmed
Professor John Stradling
John Radcliffe Hospital
While heavy snoring is a nuisance for partners, it can be a sign of a far more serious problem for the snorer.
Sleep apnoea, in which the airways are partially or completely obstructed during sleep, stops the person getting a good night's rest, making them very sleepy during the day.
It has also been linked to a greater risk of high blood pressure and cardiovascular problems.