A robust laugh gives the muscles of your face, shoulders, diaphragm, and abdomen a good workout, and sometimes even your arms and legs. Heart rate and blood pressure temporarily rise, breathing becomes faster and deeper, and oxygen surges throughout your bloodstream. Your muscles go limp and your blood pressure may fall, leaving you in a mellow euphoria. A good laugh can burn up as many calories per hour as brisk walking. During a good hearty laugh your brain orchestrates hormonal rushes that rouse you to a high-level alertness and numb pain.
Researchers speculate that laughter triggers the release of endorphins, the brain's opiates.
This may account for the pain relief that accompanies laughter. Combinations of helpful things are not additive but multiplicative.