We exercise for different reasons: to be healthier, to be fitter, to look better, or most likely for all three reasons. But there's another reason to exercise: You'll live longer, especially if you do one simple thing.

According to research published in Progress in Cardiovascular Diseases, jogging is the best form of exercise for increasing your life span, lowering the risk of dying from any disease by 40 percent.

As the authors of the study say, "People who run tend to live about three years longer than those who don't," reducing a person's risk of premature death by between 25 and 40 percent. And that even takes into account factors like smoking and drinking, and health problems like obesity or hypertension.

Granted, that doesn't mean you should start running 20 hours a week. The study found that improvements in life expectancy top out at about four hours of running per week, hence the "three years longer" conclusion.

And of course it's not just the running that makes an impact. Any form of consistent exercise tends to result in a snowball effect: Most people who stick with an exercise program typically start eating better, too. (The same works in reverse: When you stop exercising, it's easy to stop paying as much attention to your diet.)

Don't think you can put in four hours of jogging a week? That's OK. Other research, like the Copenhagen City Heart Study, looked at healthy joggers and nonjoggers for more than a decade and determined that "the most favorable running regimen for reducing cardiovascular mortality" was six miles per week, broken down into three running days per week at a pace of seven miles per hour. (That's right at 8.5-minute miles, which for many is a little fast. But then again you only have to maintain that pace for two miles.)

But if you decide to increase your life expectancy by jogging, do yourself one other favor: Go running first thing in the morning.

Researchers at the University of Vermont found that aerobic training of "moderate intensity," with an average heart rate of around 112 beats a minute -- elevated, sure, but it's not like they were hammering away -- improved participants' mood for up to 12 hours after exercise.

"Moderate intensity aerobic exercise improves mood immediately, and those improvements can last up to 12 hours," says Dr. Jeremy Sibold. "This goes a long way to show that even moderate aerobic exercise has the potential to mitigate the daily stress that results in your mood being disturbed."

And you'll also feel smarter; exercise creates new brain cells and makes those new cells more effective. As Gretchen Reynolds says, "Exercise does more to bolster thinking than thinking does."

So there you go: Work out first thing. You'll feel better. You'll be smarter. (Sure, you could work out after work, but then the happy feelings and extra brainpower will be wasted while you're asleep.)

And best of all, you'll live longer.

Can't beat that.