The equation for losing weight is simple: Just consume fewer calories than you burn and you will lose weight. So it makes sense that finding ways to be more active will help you lose weight.

But not if you decide the best way to be more active is to get less sleep.

I have a friend who does that. He loves the math: The metabolic equivalent (MET) of sleep is .95. He weighs 220 pounds, or 99.79 kilograms. Multiply that by .95, and that by 3.5, divide by 200, and that means he loses 100 calories every hour he sleeps. (Yeah, that's pretty convoluted... so here's a calculator.)

Contrast that with the number of calories he thinks he burns while he's awake. Somehow he's decided that his activity level while awake is similar to walking -- because we all tend to overestimate our activity level -- and since the MET of a steady walk is 2.8, that means he burns 205 calories every hour he's awake.

He loves this math, too. Since 205 minus 100 equals 105, then if he sleeps two hours less every night he burns 200 extra calories every day. If he does that for two weeks, boom: He loses a pound!

Of course it doesn't actually work that way, but it does show the lengths we go to in order to rationalize our health and fitness strategies, and why we get frustrated when those strategies don't work.

For one thing, he doesn't walk constantly all day, much less during those two "extra" hours. So the difference in calories burned between awake and asleep isn't that big. For another, he's chronically tired, making it much harder for him to find the energy to go for a run or to the gym... to do things that will actually make him fitter and burn a significant number of calories.

More important, though, decreasing the amount of sleep you get can actually cause you to gain a pound in two weeks; one study found that people who get less than the optimal amount of sleep showed an increase in eating of about 300 calories per day.

That result is in part behavioral: When you're tired, you instinctively reach for something to eat -- and when you're tired you're a lot less likely to reach for something healthy, since fatigue is the enemy of willpower and smart decisions.

But that result is also hormonal. Research shows a lack of sleep increases ghrelin, a hormone that increases appetite, and decreases leptin, a hormone in fat tissue that signals when you're full. When you get less sleep your body automatically tells you it's hungrier, and does a worse job of letting you know when you're full.


Less sleep also negatively impacts your sensitivity to the insulin your body produces. Insulin turns food into fuel, and after just four nights of reduced sleep, research shows that your insulin sensitivity decreases by 30 percent, which means you don't properly use the food you consume. That can cause you to gain weight and, over the long term, lead to diabetes, heart disease, stroke, and premature death, according to the NIH.

Another yay.

So how much sleep do you actually need?

Most people are convinced they need less sleep than other people. Going on little sleep is like a badge of honor; it makes you part of the "sleepless elite." For a small percentage of the population, that might even be true.

But it isn't true for me, and it probably isn't for you.

According to Daniel J. Buysse, only 5 percent of the people who think they need just five or six hours of sleep a night are actually able to get by on that amount. The other 95 percent are chronically sleep deprived, making them part of the 33 percent of U.S. adults who get less than the recommended seven hours of sleep per night, which, clearly, makes it only harder for them to maintain a healthy body weight.

The easiest way to lose weight -- or make it less likely you'll gain weight? Get enough sleep. Here are some handy tips. If you don't know the amount of sleep that is optimal for you, start by getting seven hours a night. (After a lot of experimenting, I've found that 7.5 hours is my magic number.)

You'll have more physical energy when you work out, which means you'll burn a lot more calories -- and be a lot more likely to actually work out in the first place.

And you'll have more mental energy for those times you need to make smart eating choices.

And best of all, the science will be on your side.

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