Got more downtime now than Donald Trump has real estate? Whether you have some extra hours because of job cutback, a decision to go out less or because you've retired, you can make that time work for you by investing some of it in a longer life.

How? Give it away. Sounds counterintuitive in an era when everyone can use some extra bucks. But the payoff could be huge. In a recent study of 6,360 older people, scientists at the University of California San Francisco found that people who volunteered were half as likely to die during the four-year study as those who didn't.

Yes, half.

Plenty of research has linked "giving back" with living longer. What sets this study apart is its size and sophistication (it controlled for factors such as income, chronic health conditions, disabilities and depression). Why is the "volunteer factor" so powerful? We've found that it gives you at least four key, life-giving things:

1. A sense of accomplishment. Believing in your ability to do positive things spills over into all aspects of your life - giving you the spark to finally quit smoking, stick with an exercise program or reach for an extra serving of broccoli. Even if you don't feel all that powerful at first, volunteering arms you with the sense that you can make a difference.

2. An emotional cushion. Volunteers feel that any despair or depression in themselves can get better, something missed by those who don't help out others. These mental-health lifts can mean the difference between life and death in the months and years after a heart attack.

3. More physical activity. People who volunteer in the award-winning ExperienceCorps program, which places tutors in public schools across the U.S., burn twice as many calories per day as nonvolunteers.

4. A bigger social network. Volunteering tends to widen your circle of friends - a key measure of well-being, particularly as you age.

Even better: You can get these benefits without turning volunteering into a 9-to-5 (or -7 or -8) job. The health benefits really build up, the way interest accrues in a savings account, if you put in just two hours a week - about 100 hours per year. And if you're job-hunting, a few hours a week of volunteering could help you develop new skills or make connections that get you hired. The time away from tweaking your resume and searching online job boards will help your hunt, too.

How to make a volunteer investment you'll love, and that will love you back? Ask yourself these questions:

What's important to me? Give away your time to something you believe has meaning and purpose, and watch your happiness go up, which is critical in stressful times. You don't have an obligation to society to find a bigger purpose. You have an obligation to yourself.

How much time do I want to give? A daily commitment may be great for network-building, but once a week is fine, and once in a while is a good start.

Would I rather do something I'm good at? Donating your gardening or accounting expertise may be very satisfying. But if you want to try out a new interest or learn a new skill, go for it.

What's available near me? If you can't find a local group that works for you, try www.volunteermatch.org or www.mentoring.org, or if you fit the demographic, www.seniorcorps.org. Teens can contact us at healthcorps.org.

Got anything else? Don't let yourself get stuck in a volunteer opportunity that's not working for you. Accept that it's not right and move on. The right spot will leave you feeling like you can't wait to come back next time. Your happiness deserves a good match. So do the people you're helping.

The You Docs - Mike Roizen and Mehmet Oz - are authors of the best-selling "You: Being Beautiful." To submit questions and for more info, go to www.RealAge.com.