“My metabolism is so slow!” “She can eat whatever she wants without gaining a pound—she must have a fast metabolism.” We throw the M word around all the time when discussing food and weight loss, but there’s so much more to it than how quickly you can burn off a cheeseburger.

Your metabolism is the process that converts what you put in your mouth into the energy that powers every little (or big) thing you do. And, yes, your basal metabolic rate (BMR)—the number of calories your body needs to melt just to stay alive—is partly genetic, which explains why some women can eat junk and stay rail thin, while others seem to just look at a pumpkin pie and gain 5 pounds.

Your metabolism is also influenced by plenty of other factors, however, such as your age and even your lifestyle habits like sleeping routine and snacking patterns, says Pamela Peeke, MD, author of The Hunger Fix ($12,amazon.com).

Skimping on slumber, for example, can cause weight gain, a study from the University of Colorado shows. “Insufficient sleep raises levels of appetite-stimulating hormones, such as cortisol,” says Louis Aronne, MD, an obesity specialist at Weill Cornell Medical Center, in New York City. (Plus, the longer you’re up, the more time you have to be hungry for your next meal.)

Here are three expert rules for keeping your metabolism revved all day long.