What's more important for weight loss: Exercise, healthy eating or sleep?

For the past few decades, the question has been, "What's more important for weight loss: exercise or healthy eating?"

And, for decades, we have cut calories, fat and carbs, and spent more money on gym memberships than our parents would have ever imagined. We have also gained, not lost, weight.

We also haven't slept. While the National Sleep Foundation recommends that adults ages 18 to 64 get between seven and nine hours of sleep a night, 30 percent of American adults clock six or fewer hours, according to the U.S. Centers for Disease Control and Prevention. And people who sleep less than five to six hours a night are up to 45 percent more likely to be obese, says Alexandra Sowa, a clinical instructor of medicine at Weill Cornell Medical College in New York City. "More than one-third of Americans are obese and a similar number is sleep deprived. Both can be considered to be significant health problems, approaching epidemic proportions." So maybe there's your answer. Maybe it's sleep that's really the most important part of the weight-loss equation.

That's not to say that all three healthy habits aren't vital to helping you hit your ideal weight – not to mention cutting your risk of Type 2 diabetes and heart disease. But sleep is unique in its ability to make or break the other two.

Basically, while you can probably get pretty decent sleep even if you're eating junk food and sitting on your keister all day, you can't expect to get in a good workout or eat healthy diet if you're skimping on shuteye. That's because, not only does poor sleep drain your body of energy, it affects energy balance and function in every tissue of your body, says neurologist Phyllis C. Zee, director of the sleep disorders center at Northwestern University Feinberg School of Medicine in Chicago.

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For one, sleep deprivation throws off your body's levels of hunger-regulating hormones making binging a biological inevitability, not a matter of willpower, says registered dietitian Kelly Pritchett, assistant professor of nutrition and exercise science at Central Washington University. Specifically, it can reduce your body's levels of leptin, "the satiety hormone," and increase ghrelin, "the hunger hormone."

That's part of the reason why in one Mayo Clinic study, when men and women cut 80 minutes from their regular sleep schedule, they ate an extra 549 calories the next day. Those calories aren't coming from baby carrots, either. Previous research from the University of California–Berkeley used brain scans to find that sleep deprivation impairs activity in the brain's frontal lobe, which is in charge of complex decision-making, but increases activity in the brain's reward center, the one that lights up in response to sugar, salt and fat.

"When you are sleep deprived, your brain is more likely to want high-energy foods for fuel," Zee adds. "It shouldn't be a big surprise. When you are up at 2 or 3 in the morning, you aren't thinking about salads."

And even if you do somehow manage to stick to your healthy-eating guns while running on too-little sleep, your weight-loss progress will still suffer. In one University of Chicago-led study, when dieters got 8.5 hours of sleep a night over the course of two weeks, half of the weight they lost was from fat. However, when they only got 5.5 hours of sleep a night, their rate of fat loss dropped by 55 percent – even though they were following the same diet.

Again, the connection may come down to hormones. Research published this year in the journal Diabetologia shows that just four days of sleep deprivation reduces the body's insulin sensitivity, increasing the risk for fat storage. The study also found that sleep deprivation reduces the body's levels of growth hormone, which contributes to not only your fat-burning potential but also your ability to recover from exercise. Meanwhile, Zee's research, published in the Journal of Clinical Sleep Medicine, suggests that getting enough sleep may affect your ability to work out more than working out affects your sleep quality.