Getting too little sleep can have all kinds of negative consequences, including making you cranky and impairing your driving. A growing body of evidence suggests an addition to that list might be in order: Lack of sleep might also make you fat.

The intriguing prospect that sleep duration may play a role in how much we weigh has researchers busily conducting studies to tease out the potential relationship between shut-eye and BMI.

Although it might seem intuitive that under-sleeping leads to overeating, science hasn’t yet found a direct cause-and-effect relationship between lack of sleep and being overweight. “There is a very, very strong link,” says Jim Hill, director of the Colorado Nutrition Obesity Research Center at the University of Colorado and a spokesman for the American Society for Nutrition. “People with sleep problems tend to have obesity. Why? That’s where the research is.”

In one of the latest studies, Marie-Pierre St-Onge of the New York Obesity Nutrition Research Center at St. Luke’s-Roosevelt Hospital found that sleep-deprived people tend to burn the same number of calories — in her study, about 2,600 per day — as people who enjoy a full night’s sleep. But her research, published in the August issue of the American Journal of Clinical Nutrition, found that sleep-deprived subjects consumed about 300 more calories per day (2,800 vs. 2,500) than the well-rested subjects. Because it takes just 3,500 calories to add a pound to your body, St-Onge says, “if people kept that up for a while, it would add up really, really quickly.”

Still to be determined: Do sleep deficits actually cause people to become overweight? Or does being overweight cause people to not get enough sleep? Or might the two conditions share some underlying factor that’s not been found?

(WP photo illustration)

Michael Breus knows those questions need answers. But he’s not waiting for all the dots to be connected. Breus, a prominent sleep disorders specialist, has recently published “The Sleep Doctor’s Diet Plan: Lose Weight Through Better Sleep” (Rodale/Mindworks, 2011). As the subtitle suggests, he’s convinced that the body responds to too-short sleep by packing on pounds — and, conversely, getting more sleep is an effective weight-loss strategy. (Although the book touts Breus as the Sleep Doctor, he is a psychologist with a PhD, not a physician.)

Breus spells out (and supports by citing research) several means by which too little sleep could lead to weight gain. When you’re sleep-deprived, he says, your body moves into a different mode. “On a physical level, the key things are hormones,” Breus says. When you lack sleep, he explains, “your metabolism slows down. Your body is trying to conserve energy stores” to carry you through the longer period of wakefulness. That slowdown triggers the release of cortisol, a hormone that your body secretes in response to stress and that boosts your appetite: Your body senses it needs more energy, so it demands more food.

At the same time, Breus says, sleep deprivation causes your body to release more ghrelin, the hormone that signals hunger, and less leptin, the hormone that tells your body it’s full. When those hormones are out of whack, your body wants more food and lacks the sensitivity to know when to stop eating. Combined with the fact that you’re awake for more hours a day, that hormone cocktail can send you snacking into the wee hours.

“The later you’re up at night, the greater the likelihood that you’re going to eat,” Breus says. And “you’re more likely to eat high-fat, high-carb foods.” Such “comfort foods,” he says, “literally act like calming agents by increasing serotonin,” a hormone that promotes calm and contentment.

And that’s just the hormones. Breus notes that a healthful night’s sleep — about 7.5 hours for most adults — provides opportunity for your body to enjoy five 90-minute sleep cycles, each of which includes a deeply restful rapid-eye movement, or REM, phase. As he explains it, those cycles include increasing amounts of REM as your sleep progresses, so losing out on one or two sets of REM at the end of your sleep time digs deep into your total REM time. As it happens, Breus says, you burn more calories during REM than in other parts of the sleep cycle. Those unburned calories, he says, can add up to weight gain.

Hill agrees that lack of sleep can “really screw up the whole neuroendocrine chain,” making you eat more by “disrupting the hormones that control hunger and satiety.”

But he urges caution. “I’m convinced [sleep deprivation and obesity] are linked, but I don’t believe the science has proven it.

“We have to be very careful that we’re not giving the wrong message,” Hill says. “I don’t think it’s going to be as simple as you fix the sleep, you fix the obesity.”

Tips for a sound (and slimming?) night’s sleep

Whether or not science ultimately proves that lack of sleep contributes to being overweight, most of us could benefit from catching more Z’s. Sleep disorders specialist Michael Breus suggests these (mostly) simple steps to sounder sleep:

3Pick a sleep schedule and keep it consistent. “If your body knows when to go to bed every single night, it does it, and does it well.”

Exercise daily. “Exercise helps to reduce anxiety,” one of the main causes of sleep loss. ●But don’t exercise too close to bedtime; stop four hours before lights out.

Keep a worry journal. Breus says writing down your worries can reduce anxiety’s grip.

Limit pre-bedtime activity. “The time right before bed should be spent doing three things: the stuff you need to do to get ready for the next day, such as getting the kids’ backpacks ready; personal hygiene; and relaxing time.”

Don’t consume caffeine after 2 p.m. Caffeine can keep you awake eight to 10 hours after you ingest it.

Stop imbibing three hours before lights out. “Alcohol may make you feel sleepy, but it keeps you out of that deeply restorative stage of sleep.”

●First thing in the morning, get 15 minutes of sunlight. “That’s the easiest way to reset your circadian rhythm,” the internal system that regulates your sleep.

A sleep-inducing smoothie

This recipe from Michael Breus’s book uses dairy’s natural relaxant properties and an ideal carb-protein ratio to make you drowsy when consumed shortly before bedtime:

• 1 container (6 ounces) low-fat vanilla yogurt

1 / 2 ripe banana, coarsely chopped

1 / 2 cup fat-free milk

1 / 4 teaspoon pure vanilla extract

1 / 4 graham cracker, crushed

Combine the yogurt, banana, milk and vanilla extract in a blender. Blend until smooth. Pour into a glass. Sprinkle with the graham cracker.

Serves 1

Per serving: 249 calories, 2.5 g fat (1.5 g saturated), 11 mg cholesterol, 175 mg sodium, 45 g total carbohydrates (37 g sugars), 1.5 g fiber, 13 g protein