When we laugh heartily among friends, we exhale repeatedly without drawing breath. This physical effort leaves us exhausted and triggers the release of protective endorphins, one of the complex neuropeptide chemicals of the brain that regulate pain and promote feelings of well-being. Watching 15 minutes of comedy in the company of others can increase your pain threshold by an average of 10%. Laughter is 30 times more likely to happen when we are with others than when we are alone.
Proceedings of the Royal Society, Dec 2011