Nature’s incentive programme still isn’t fully understood. Greg Gerdeman, an assistant professor of biology, explains the current theory.

1/ Get In The Groove

When you run past your comfort level (i.e. after 30 minutes at about 80% max heart rate), special cells secrete two mood modifiers: endorphins – which are opioids – and anandamide, which is similar to THC, the active ingredient in marijuana.

2/ Endorphins Come Knocking

Your blood carries the two chemicals to your blood-brain barrier, the gatekeeper to your brain cells. Anandamide crosses easily. But endorphins are large and few make it in. No problem, though: your brain also produces endorphins. Both chemicals boost your mood – and perhaps your mileage.

3/ Brain: What Pain?

The two mood boosters are now tapped into your central nervous system, where they limit the signal- ing power of pain sensors called nociceptors that spread out from your spinal cord. Result: you don’t realise how much your legs are burning.

4/ Endorphins Kick In

The endorphins downshift your brain’s prefrontal and limbic regions, which regulate your emotion and motivation. You begin to feel calm and comfortable. Then, as more endorphins reach these areas, you may even start feeling euphoric.

5/ It’s High Time

Anandamide triggers dopamine, a neurotransmitter that contributes to lust, pleasure and addiction. It also binds to cannabinoid receptors in your brain’s pleasure circuit, where it sends bliss vibes for minutes or even hours. (And no intense munchies!)