Avid exercisers often enjoy a euphoric feeling after their workouts. Sometimes called a “runner’s high,” this notable post-workout boost in happiness and energy levels is what keeps many devoted exercisers coming back for more.

The feel-good vibes are typically attributed to endorphins, which are neurochemicals produced in your brain’s hypothalamus and pituitary gland. These natural painkillers are similar in structure to the drug morphine and activate opioid receptors in your brain to help minimize pain.

Endorphins are linked to your body’s reward circuits as well and are associated with other feel-good activities like maternal behavior, eating and drinking or having sex.¹

Endorphins in your blood also increase in response to pain and stress, such as exercise, and it’s long been suggested that these endorphins help people feel good after their workouts. There’s even a term, “endorphin junkie,” often used by gym rats who are “addicted” to the exercise “high.” There’s just one problem.

Endorphins Can’t Cross Your Blood-Brain Barrier

The idea that endorphins cause the runner’s high has been in circulation for decades. Dr. J. Kip Matthews, a sport and exercise psychologist, told CNN:²

“Long-distance running was quite popular in the mid-1970s around the same time that endorphins were discovered. Anecdotally, there were a lot of reports of the so-called ‘runner’s high.’

… By suppressing the experience of pain, a number of researchers put forth the idea that endorphins could be the source of this euphoric feeling after intense exercise.”

However, as CNN reported, research has shown that endorphin levels might not increase in your body until you’ve exercised for a full hour.³

So why you can still feel euphoric after a very short, intense workout like high-intensity interval training (HIIT)? Some research suggests endorphins are only produced during anaerobic exertion, such as HIIT or intense weight training (and not during typical aerobic exertion unless you exercise for about an hour).⁴

This still doesn’t explain the full picture, however, as most related studies have measured endorphin levels in your blood during exercise. This isn’t indicative of the endorphin levels in your brain, because endorphins can’t cross your blood-brain barrier.⁵

So while exercise may increase blood levels of endorphins, it’s the endorphins in your brain that would make you feel good. And research hasn’t proven that exercise increases brain levels of endorphins.

What does increase in your brain following exercise is a neurotransmitter called anandamide, sometimes known as the bliss compound.