When you laugh, your brain squirts out endorphins, which are chemically similar to heroin, morphine and cocaine. You can think of endorphins as free, legal, totally natural and safe opiates that reduce pain, make you feel good and impart a tranquilizing effect on your body. Further, endorphins boost your immune system, which helps you better thwart sickness and disease.
From Why I’m a Laughter Junkie
by Marie Overfors.