While most runners are chasing after a new goal or a personal best, some are chasing after something slightly more elusive: a runner’s high.

Some describe it as euphoria. Others as calm, clarity or even a sense of being completely absorbed in the moment. Many people experience some version of it after a long run or a spin class.

For years, scientists believed the phenomenon was caused by endorphins, the body’s natural opioids. But more recent neuroscientific research now indicates that this might not wholly be the case.

Instead, a runner’s high appears to emerge from several brain systems working together. This explains why a runner’s high has so many far-reaching effects – and why these effects can often persist long after the initial experience of euphoria.

The brain’s own cannabis system

Endorphins are naturally-occurring brain chemicals which are released during periods of physical stress help to reduce pain.

Aerobic exercise increases circulating endorphins. Numerous studies have demonstrated higher levels of the endorphin beta-endorphin following prolonged aerobic exercise, such as running. This is why endorphins were long believed to be the sole cause of a runner’s high.

But there’s a key problem with the endorphin theory. This is because endorphins circulating in the blood struggle to cross the blood-brain barrier – the tightly regulated system that protects the brain from substances in the bloodstream.

So while endorphins may well........

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