Air:We can't see it, and we don't think about it. But each of us breathes about 5,000 gallons of the stuff every day, and without it, we'd survive only 8 short minutes.

It's also one of the keys to staying young and healthy.

All day long our muscles and organs get a minimal amount of oxygen as we breathe normally, but if we want to take advantage of oxygen's anti aging effects, we need to get a little extra. It turns out that the best way to do that is with aerobic exercise. That means huff and puff movement, like brisk walking, swimming, biking, and hiking.

When we exercise aerobically, our muscles demand more oxygen and blood than when we're just sitting on the sofa watching television. To fill the demand, our hearts beat faster and stronger, and we start to breathe more heavily.

Cash In on the Benefits

All that huffing and puffing along with everything else that happens when we exercisedoes us a great deal of good. It's like a low risk investment that yields tremendous short and long term profits. Here are some of the immediate youth enhancing benefits exercisers can cash in on.

Boosts metabolism. All that heart pounding, lung filling exercise burns a lot of calories and elevates our metabolism, says Miriam E. Nelson, Ph.D., director of the Center for Physical Fitness at Tufts University School of Nutrition Science and Policy in Boston and author of Strong Women Stay Young.

As women, we can use all the help we can get. When we hit our thirties, our metabolisms begin to slow by 2 to 5 percent per decade.

Our metabolic rates are already 10 to 12 percent lower than men's. That's partly because pound for pound women have more fat and less muscle than men and fat burns virtually no calories. Muscles, on the other hand, burn lots of calories as they contract and stretch, making them our metabolism's best buddy.

Why is our metabolism so important? Because it's what helps us control our weight. As it slows, so does our body's ability to use up the calories we eat before they're converted to fat, Dr. Nelson says. Exercise for at least 30 minutes every day, and you'll maintain or even lose weight by giving your metabolism a daily boost.

Boosts energy. Try this the next time you're falling asleep at your desk: Go take a brisk 10 to 15 minute walk. Chances are that you'll feel refreshed and energized when you return. "After it's over, you feel like your energy level is really surging," says John Duncan, Ph.D., an exercise physiologist at Texas Woman's University Center for Research on Women's Health in Denton. A number of things probably go on in your body to create that energy boost, he says. One is that your brain releases feel good chemicals called endorphins the same ones that, in excess, create the "runner's high" that marathoners often experience.

Reduces stress. Studies show that exercise is a great stress buster. And the best part is that you don't have to run a 3 minute mile to take that load off your shoulders. Researchers at the University of Georgia in Athens found that anxious college women cut their anxiety in half just by leisurely riding an exercise bike for 20 minutes.

Makes failing asleep E Zzz. If you've been counting more sheep than a shepherd lately, you're not alone. Women age 40 and older are especially prone to insomnia as they begin to experience the hormonal changes that usher in menopause. Aerobic exercise can improve your sleep by reducing stress, tiring you out, and regulating your body temperature.

The best time to exercise for improved sleep is in the late afternoon, according to Peter Hauri, Ph.D., co director of the Sleep Disorders Center at the Mayo Clinic in Rochester, Min nesota. The body goes through a cycle of rising and falling temperatures throughout the day. When your temperature is at its lowest point, it's easiest for you to fall asleep. Vigorous exercise in the afternoon can boost your body temperature for up to 5 hours, so your temperature will drop just in time for bed.

The worst time to work out is less than 11/2 hours before you normally hit the sack, when your body temperature will still be elevated. But everyone is different, adds Dr. Nelson. As long as you cool down adequately before tucking yourself in and you don't have problems sleeping, exercising at night is fine.

Revs up your sex drive. If your libido is in low gear, exercise may give it a turbo boost. Experts say that aerobic exercise can put the sizzle back in your sex life in a number of ways. First, it reduces stress.

When we're more relaxed, we're often more interested in having sex, says David Case, Ph.D., a research specialist in the department of psy chology at the University of California, San Diego.

Exercise can also make you feel better about your body as you find yourself becoming more fit. The more attractive we feel, the friskier we usu ally are, he says. And finally, exercise has been found to boost the levels of the hormone responsible for sex drive in men, according to a study done by Dr. Case and colleagues at the Univer sity of California, San Diego. And that effect may be similar in women, Dr. Case says.

Eases menstrual cramps. When cramps hit, you're probably not in much of a mood for a jog. But women who exercise regularly experience fewer and less painful menstrual cramps. "We're not sure exactly how exercise helps, but it may be that fit women have tighter abdominal muscles, and that may be beneficial somehow," says Mary Lang Carney, M.D., medical di rector of the Center for Women's Health at St. Francis Hospital in Evanston, Illinois. Exercise also relaxes us and produces those "happy hor mones" called endorphins, which may help relieve the discomfort as well.

Treats you to a natural facial. Ever hear the term pregnant glow? Well, exercise can give your face that same rosy radiance. The glow probably occurs after exercise because of the extra blood your heart pumps throughout your body, explains Priscilla Clarkson, Ph.D., professor of exercise science and associate dean of the University of Massachusetts School of Public Health in Amherst. What's more, women who exercise regularly may feel better about themselves. And when you're happier, your face tends to exude that charisma, she says.