The runner's high is as mysterious as it is delightful. Some athletes claim to feel it every time they exercise, whereas others insist it's only a once-in-a-lifetime experience. But just what it? And more importantly, what causes a runner's high?

What is a runner's high?

Since the 1970s, conventional wisdom has held that the feeling is the result of a rush of neurochemicals called endorphins. Since endorphins attach themselves to receptors in the brain associated with pain relief, runners felt a high similar to that of morphine.

For years, though, scientists doubted endorphins' role. The chemicals may have shown up in a runner's blood after exercise, but they believed that the molecules were too large to pass through the barrier between the cardiovascular system and the brain, making any effect on pain receptors unlikely. However, in 2008, German researchers used newly developed chemicals to detect the presence of endorphins in the brain with a PET scan. Comparing images taken before and after a two-hour run, the researchers found that endorphins not only were present, but also had attached themselves to parts of the brain associated with emotions. Runner's high wasn't a shot of morphine—it was literally a love of running.

Still, more recent studies have altered even that theory. It now seems likely that the high results from a cocktail of multiple neurochemicals, each of which moves along its own neural pathway. One possible culprit is the chemical anadamide, which in a 2012 study was found in the bloodstreams of both humans and dogs after exercise. This suggests it may have played an evolutionary role in developing humans' and dogs' distance-running and frisbee-chewing abilities, respectively.

How to get a runner's high

• Run. Or don't! Despite the name, runner's highs can lighten the load for anyone performing repetitive, endurance-based exercise of adequate intensity.

• Give it time. Though there's no precise formula, it should take at least 20 to 30 minutes of exercise to get sufficient quantities of neurochemicals moving.

• Keep a good pace. The most difficult part of attaining a runner's high is hitting the right window of intensity. If you don't push hard enough, your body won't be prompted to produce a neurochemical response. Go too hard, though, and the physical discomfort of maintaining your effort will counteract the bliss of the high.

• Keep yourself in shape. Because there's a minimum threshold of exertion, beginners may have to work up to the kind of workout that can produce a high.