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Melissa Miller was never a great sleeper. Most of her life, the 41-year-old marketing director managed with just a few hours of shut-eye each night. But after 9/11, things got worse and she noticed an unfortunate change: "I started putting on weight." Twenty-five pounds, to be exact. "I didn't have the energy to exercise, and I was eating more carbs. I was craving them." She finally sought help from New York City nutritionist Oz Garcia, Ph.D. "The first thing he did," she remembers, "was stabilize my sleep."

How is this possible? Shouldn't more waking hours equal more calories burned? Actually, no. "Sleep is a very dynamic aspect of our day," says Garcia. "It isn't just a time when the system shuts down and nothing occurs. The opposite is true; a lot is occurring. The body reboots every aspect of metabolic function. The brain produces critical messenger molecules, or neuropeptides, that play a vital role in weight regulation. If you don't get seven to nine hours of sleep regularly, there appears to be a cumulative effect. After a while, those neuropeptides — and beyond that, neurotransmitters — can't do their job."

The science of sleep: How does it work? In her book Sleep Away the Pounds, nutritionist Cherie Calbom explains the delicate chemistry. "When you don't sleep enough, your level of ghrelin — the munch hormone, which triggers appetite — goes up," Calbom says. "At the same time, your level of leptin, which controls your appetite and aids in the metabolism of carbohydrates, goes down." When leptin levels drop, we crave more food, particularly carbs, whether or not we've already consumed enough calories.

What if I'm fighting my cravings and eating a low-calorie diet? Will lack of sleep still make me gain weight? Possibly. Sleep deprivation also throws other hormones key to weight management out of whack. "The whole endocrine system can get off balance," Calbom says. Unstable insulin levels can lead to increased appetite, poor sugar metabolism, hypoglycemia, and possibly type 2 diabetes. Dips in growth hormone may promote weight gain. "The adrenal glands may start pumping out cortisol at the wrong time of the day, and excess cortisol can cause fat to be deposited, especially on your midsection," Calbom continues. "I'll often hear clients say, 'I've been eating really well, but not only am I not losing weight, I'm putting on pounds.' Lack of sleep is often a culprit."

Who has the time for all that sleep? Find the time. Go to bed an hour earlier and sleep a little later. And it's not just quantity but quality that counts. "It's important to have restful, restorative sleep," Garcia says, cautioning that you won't get it if your body is busy digesting food and/or alcohol at the same time. "When your body is going to sleep, it wants to sleep, not reawaken all the systems that break down food." He recommends keeping alcohol to a minimum and not eating for two or three hours before bedtime: "Otherwise, it causes a very light sleep through the night in which your organs don't get a break" and your blood sugar is on a roller-coaster ride.

What if I can't sleep? You're not alone. More than 60 million Americans suffer from a variety of sleep disorders. "Insomnia is believed to be as much as three times more common in women," says Michael Twery, Ph.D., director of the National Center on Sleep Disorders Research. See your doctor to determine whether there is a medical cause that can be treated.

Can't I just take a sleeping pill? That will help you physically log the requisite hours in bed, but it might not give you the quality of sleep you need to manage your weight. "Sleep drugs act like neurological sledgehammers," Garcia says, by inhibiting the body's ability to restore itself.

So how can I catch more z's? Calbom and Garcia agree: Reduce or eliminate caffeine, sugar, and alcohol. No late-night meals. No TV just before bed. Warm baths can help, as can meditation and deep-breathing exercises. Garcia often prescribes 5-HTP, calcium, and magnesium.

Does sleep really help shed pounds? Absolutely, says Miller, Garcia's client. "I'm craving less carbs. Exercise is less of a chore. I'm eating more than I did in my 20s and 30s but better-quality food." Best of all: "I've lost 24 pounds."

What do you think?