As a result of these and other studies, researchers began to theorize that getting more sleep just might be the answer to society's burgeoning waistline. But before you trade the cost of your gym membership for a pricey new mattress, take note: Experts also say the relationship is not as obvious as it seems.

The reason: Enter the somewhat mysterious nocturnal ailment known as "obstructive sleep apnea." People with sleep apnea may stop breathing for up to a minute, sometimes hundreds of times during the night while sleeping, says Dominic Roca, MD, director of the Connecticut Center for Sleep Medicine at Stamford Hospital.

Though the exact cause of the problem remains unknown, Roca and others believe that in most instances physical abnormalities inside the mouth and neck cause the soft tissue in the rear of the throat to collapse. This briefly closes off air passages many times during a night, causing disruption in breathing and a tendency to snore.

The end result: Although you may go to bed early and think you are getting a good night's rest, the disruption in breathing prevents you from getting deep sleep. Eight hours of disrupted shut eye can leave you feeling like you had only four.

"You wake up feeling tired and continue to feel tired all day," Roca tells WebMD.

The Link Between Sleep Apnea and Weight

So what does sleep apnea have to do with weight gain?

First, says Roca, patients who suffer from sleep apnea are more likely to be obese. However, studies show they do not have the usual low leptin levels associated with being overweight. In fact, Roca says that folks with sleep apnea have uncharacteristically high levels of leptin.

What's more, when their apnea is treated, leptin levels drop -- and somehow that helps them to lose weight.

"I've had about thirty patients who, when successfully treated for their sleep apnea were able to lose weight -- possibly because they had more energy, so they were more active and they just ate less," says Breus.

So why does low leptin seem to cause weight gain in some folks while allowing others to lose weight? One theory says that it may not be the level of this hormone that matters so much as a person's individual response to it. In much the same way that obese people can become resistant to insulin, folks with apnea may be resistant to the fullness signal that leptin sends to the brain.

"It's like the body is trying to tell them to stop eating, but their brain just isn't getting the message," says Breus.

Another theory: The overall response to leptin may be more individual than we think. Experts say our environment, dietary habits, exercise patterns, personal stress levels, and particularly our genetics may all influence the production of leptin and ghrelin, as well as our response to them.

The fact that we just don't know causes at least some experts to view all the research on sleep and weight with a cautious or skeptical eye.

"There is a serious challenge to the closing of the loop. That isn't to say that what we know about leptin and ghrelin is not important, or that when we finally do understand it that it won't be crystal clear -- but right now it just isn't," Rapoport tells WebMD.

Breus agrees: "I think we are likely to find that bad sleep matters but that it's likely to be bad sleep plus some other problems. I don't think we know what they are yet."

Sleep: You Can't Lose

Until doctors do know more, most experts agree that if you are dieting, logging in a few extra hours of sleep a week is not a bad idea, particularly if you get six hours of sleep or less a night. You may just discover that you aren't as hungry, or that you have lessened your craving for sugary, calorie-dense foods.

"One thing I have seen is that once a person is not as tired, they don't need to rely on sweet foods and high carbohydrate snacks to keep them awake -- and that automatically translates into eating fewer calories," says Breus.

If, on the other hand, you already sleep a lot, or you increase your sleep and feel even more tired, you should talk to your doctor. Experts say you may be one of the thousands of people with undiagnosed sleep apnea.

Says Roca: "As research continues, more and more data comes to the forefront to suggest that you simply can't cut back on sleep without paying some price."

Published Feb. 4, 2005.

SOURCES: David Rapoport, MD, associate professor and director of the Sleep Medicine Program, NYU School of Medicine, New York City. Michael Breus, PhD, Atlanta School of Sleep Medicine; clinical director of The Sleep Disorder Centers of Southeastern Lung Care, Atlanta. Dominic Roca, MD, director of the Connecticut Center for Sleep Medicine, Stamford Hospital. Annals of Internal Medicine, 2004; vol 141: pp 846-850. PLOS Medicine, December 2004; vol 1. Journal of Clinical Endocrinology and Metabolism; vol 89: pp 5762-5771. American Journal of Physiology -- Heart and Circulatory Physiology, July 2000; vol 279: pp H234-H237.

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