The term "runner's high" hit its stride, so to speak, in the 1970s, not long after it appeared in Jim Fixx's The Complete Book on Running . Runners had long claimed that some people experience euphoria while working out, but no one had named the sensation. These days, runners aren't the only ones fascinated by the "high." Researchers are, too.

The runner's high is usually characterized as an unexpected boost in energy levels, accompanied by joy and hyper-awareness. It seems to occur about 30 minutes into a workout and has been compared to spiritual awakening or sexual orgasm. Some people question whether the runner's high is real, however, and scientists still can't confirm its existence or explain how it works. Still, most of those who are trying favor one of two hypotheses.

Hormones are one possibility. Exercise seems to counter the effect of hormones that can cause stress or irritability. Studies have also shown that exercise causes the release of endorphins, hormones that can produce feelings of euphoria and have analgesic properties similar to morphine. And, researchers are exploring the link between exercise and serotonin, a brain chemical that may affect mood and behavior, appetite, and sleep patterns.

Other researchers argue that the runner's high is psychological. They note that people who exercise often report feeling better about themselves, and they suspect there's a link between runner's high and this boost in self-esteem. One such researcher had groups of women ride a stationary bike. He told one group that they were in good shape, and the other group that they were unfit. More of the women who had been told they were fit reported feeling better during a follow-up workout a week later.

Only 30 percent of runners, and 10 percent of all exercisers, are estimated to experience the high regularly. But the lucky few say it comes on when you least expect it, and that's one more reason to keep pounding the pavement.