Summer is almost here which means beach trips, pool parties and weddings nearly every weekend. You want to look your best so your instinct is to increase the length of your workouts. This is a good idea, right? Not exactly says Debi Silber, M.S., R.D., W.H.C., F.D.N. (that's a lot of letters after her name). “We know so much more about fitness now," she says. "We used to think long, drawn-out cardio sessions were the best way to blast fat. Now we know the shorter bursts in high-intensity interval training burn so much more fat in a fraction of the time." The science backs this up. Not only does high-intensity interval training (HIIT) amp up your calorie burn, it strengthens your heart, helps regulate your blood sugar, and keeps your metabolism chugging along at a higher rate after the workout’s done. It seems like longer workouts would be the logical route but there are a few reasons why it isn't. First, working out gives you feel good endorphins but extended workouts can actually raise your levels of the stress hormone cortisol. “Your body perceives long workouts as a stressor, so your adrenal glands respond as if your body’s in crisis and flood you with cortisol,” says Silber. When your body is getting stress signals, it doesn't properly convert food into glycogen or energy. Instead the calories go straight to your midsection where they are stored as fat. What's more, longer workouts can lead to injury which will take you away from your fitness routine completely. “If you’re doing the same kind of exercise for a long time, you’re putting way too much stress and tension on your muscles, joints, and ligaments,” says Silber. HIIT workouts don't leave you as vulnerable as they give your muscles a chance to bounce back. Longer workouts make it may exhaust your muscles to the point where they cannot change as easily. “With the right kind of exercise, you’re breaking down the muscle in order to build it back up," says Silber. "When you over-exercise, you’re not giving your muscles a chance to repair." Losing marathon workouts for HIIT promotes muscle confusion which protects your muscles from getting comfortable in one type of workout and not changing. The best part of HIIT? It takes less time! “The biggest excuse people make for not exercising is that they don’t have time," says Silber. "If you try to lengthen your workout, you just won’t want to do it." Try making excuses now. There is no way you can't find 20 minutes, especially when you know you are going to get even better results.