The same thing that gives marijuana smokers their high is the same thing you get from a great workout.
We read about this in Prevention magazine - And the study looked at 24 men who were either asked to run, ride a bike, or sit around for 50 minutes. Then blood samples were taken. In the exercisers, they found high blood levels of cannabinoids. And very low levels in the couch potatoes.
The lead researcher, Dr. Arne Dietrich, believes the body releases those "high" inducing chemicals to cope with the stress of exercise. And he thinks that's important for people who don't work out very often. A lot of times people quit exercising after only a few minutes because they feel uncomfortable. But the doctor suggests that people who don't like to exercise should give their body enough time to release the chemicals so they feel better during and after exercise. So how long do you need to exercise before you feel the buzz? 20 minutes of aerobic effort will trigger the exercise high.