High..

Runner's high is a pleasurable and euphoric feeling that some people experience during or after prolonged aerobic exercise. It is thought to be caused by the release of certain chemicals in the brain, including: 

Endorphins: These natural painkillers reduce pain and produce feelings of well-being.  

Endocannabinoids:  

These chemicals are similar to marijuana and have mood-altering effects.   

The idea behind runner's high is believed that exercise triggers the release of endorphins and endocannabinoids in the brain. These chemicals cross the blood-brain barrier and interact with receptors in the reward system, leading to feelings of euphoria, pleasure, and reduced pain. Conditions for Runner's High are 

Engaged in prolonged aerobic exercise (e.g., running, cycling, swimming)

Run at a moderate intensity for at least 45 minutes

Have a history of running or other regular exercise

Have a positive attitude towards exercise.