Where does
a runner's high
come from?

The full narration of the video.

That floaty feeling on a run probably doesn't come from endorphins. Stranger still: push too hard, and the good feeling doesn't show up. You've probably heard it: run long enough, your body releases endorphins, and you feel great.

Running really does raise endorphins in your blood. But they're too big to get into the brain. Think of the brain as a room with the door shut, and only a thin gap under the door. Anything in the blood that wants in has to squeeze through that gap.

Endorphins are big cardboard boxes. A whole pile arrives, and they all get stuck outside. So how does the room end up floating? Someone ran an experiment to find out. A group of regular runners each ran forty-five minutes on a treadmill.

Before the run, half of them took a pill that blocks the spots where endorphins land in the brain. The other half took a fake pill that looked exactly the same. If the good feeling were all endorphins, the people on the real pill shouldn't have felt it. But after the run, both groups felt just as happy and relaxed.

The endorphin route was blocked, yet the feeling was still there, so something else is doing the work. In the runners' blood, the researchers found something else that rose sharply after the run. It's like a little slip of paper: thin and slippery enough to slide under the door.

Its real name is endocannabinoids. "Endo" means your body makes them itself. "Cannabinoid" means they're shaped like the relaxing compound in cannabis. Your brain already has places built to catch them. That door is a filter wrapped around the brain, called the blood-brain barrier. One blood draw shows whether the slips went up.

Once the slips get inside, you feel less tense, your legs ache less, and your mood drifts up. The brain makes a little endorphin of its own, which may help a bit. But right now, the paper slips look like the main act. Still, you don't get these slips from just any kind of run.

A slow walk barely raises them. An all-out sprint doesn't do much either. They climb most at the in-between pace: breathing hard, still able to talk in full sentences, but not able to sing. That's why going too fast actually kills the high.

Why would the body work this way? One idea: long ago, people had to run long distances to find food. Dogs are great distance runners too, and their slips rise after a run. Ferrets don't run far, and theirs don't rise. Maybe the body uses these slips to reward you for keeping going. That's only a guess; nobody can go back in time to check.

And not every run brings the high. The experiments all ran for half an hour or more, and some runners go years without ever feeling it. Next time you run, find the pace where you can talk but can't sing, and keep it up for half an hour. Come back and tell us which minute the float kicked in.