Snoring can have knock on problems during the day
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.
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.
Scientists have been probing the link between weight gain and sleep apnoea, and the team from the University of California, San Francisco, measured the number of calories burned "at rest" by 212 patients.
On average, the volunteers expended 1,763 calories a day this way, but those with the worst apnoea symptoms expended 1,999, while those with the mildest form of the condition expended an average of 1,626.
The extra calories consumed are the same as a vigorous 30-minute workout in the gym.
The authors of the study, published in the journal Archives of Otolaryngology - Head and Neck Surgery, led by Dr Eric Kezirian, suggested that energy used by the nervous system as it responded to the poor quality sleep patterns of heavy snorers might be to blame.
However, he said it did not help explain why being obese and having apnoea went hand in hand.
Professor John Stradling, a sleep expert from the John Radcliffe Hospital in Oxford, said that the study findings were "entirely plausible", and fitted the experience of his patients, who found it hard to lose weight after their sleep apnoea symptoms had been relieved.
He said: "I used to joke that sleep apnoea was free exercise at night - it's nice to have that confirmed."
There were three reasons why sufferers might burn more calories at night, he said.
Firstly, they spent less time in deep sleep, where the body temperature naturally drops, they might expend more energy just struggling to breathe, and each time their sleep was interrupted by breathing problems, their body would fire a dose of adrenaline, burning up yet more calories.
However, he warned that the powerful effects of sleep apnoea meant it was not a way to lose weight.
"If you have sleep apnoea, you are very sleepy during the day, and demotivated to do any exercise - we also know that sleep deprivation increases appetite and decreases willpower."