In addition to improving aerobic fitness, interval training can burn more calories in a shorter time. There's also some evidence that intervals trigger a mechanism that helps the body burn more fat and possibly even boost metabolism more for some period after the workout ends. For the already fit, the key to getting these benefits is to go hard during those intense periods. "In my 30 seconds, I'm grabbing onto the treadmill, and I'm out of breath," says Ratey. (You shouldn't be able to carry on a conversation during the sprints.) High-intensity exercise is rougher on your body, so limit intervals to twice a week, and take a rest day in between. And be careful of overuse injuries, which tend to crop up during intense exercise. To round out your regimen and meet experts' recommendations, also do some steady aerobic activity one or two days a week.
Intervals aren't just for hard-core exercisers. They've even been used (under careful medical supervision) with heart patients. One study published last summer in Circulation found that interval training improved aerobic capacity and other markers of health for heart failure patients. And Darren Warburton, an exercise scientist at the University of British Columbia, separately found that interval training aided the health of patients with coronary artery disease. He's now investigating whether the same holds true for type 2 diabetics. Still, if you're not accustomed to exercise at all, are older than 60, are at risk of heart problems, or have arthritis, consult a doctor before you start.
For those already exercising regularly, there are simple ways to incorporate interval training into a workout. "If your mode of exercising is walking around the block, walk faster in between two light poles and then back off for the subsequent two light poles," says Gibala. The same concept works in the pool, on a bike, or on a rowing machine.
Even on days devoted to strength training, there are ways to cut workout duration. If you go to a gym and can't avoid its busiest times, learn how to use all the equipment. That will give you more options if your usual machine or weight bench is occupied, says Valerie Waters, a trainer whose clients include the überfit Jennifer Garner. If you're unfamiliar with what's there, book one or two sessions with a trainer and indicate that you want in your limited time together to learn how to use the equipment, she says. Moreover, don't be intimidated into waiting around for a machine when someone else is just sitting there sweating on it. "It's absolutely appropriate to ask, 'How many more sets do you have?' and if they say three more, it's completely appropriate to say, 'Can I work in?'" says Waters.
Go low-tech. Better yet, find yourself a corner of the gym—or your home—and just use free weights and your own body weight. Free weights torch more calories and work more of your body, especially if you use the larger leg muscles. "Doing a squat with 10-pound dumbbells is better than sitting in a leg extension machine," says Equinox's Espel. And focus on moves that use more than one joint and more than one muscle group, like a squat that goes immediately into a shoulder press, says Waters.
She and other trainers are also big fans of mixing aerobic and strength training to get a combined workout. Such "circuit training," in which you move through a series of vigorous exercises with little or no rest in between, "lets you knock out two things at once," says Durkin. You can set up your own circuit, perhaps with advice from a trainer, by intermingling resistance exercises with stints on a bike or treadmill. Many gyms have circuit training classes, too. Most also have shorter, 30- or 45-minute classes that are designed to incorporate both cardio and strength training; you can let the instructor do the planning for you.
Whatever you're doing, don't skip the cool-down to save time. Your body needs a gradual ramping down of exercise to return it to its normal state, and stopping on a dime can cause blood to pool in your legs and make you dizzy. But you don't need to stretch, says Ingraham; it doesn't work any better after exercise than before. So save the extra five minutes for thinking about what you'll accomplish with tomorrow's workout.