The chemicals are called Endorphins - They are talked about in this weeks news letter.
Here is the paragraph
Laugh out loud.
Just anticipating a happy, funny event can raise levels of endorphins and other pleasure-inducing hormones and lower production of stress hormones. Researchers at the University of California, Irvine, tested 16 men who all agreed they thought a certain videotape was funny. Half were told three days in advance they would watch it. They started experiencing biological changes right away. When they actually watched the video, their levels of stress hormones dropped significantly, while their endorphin levels rose 27 percent and their growth hormone levels (indicating benefit to the immune system) rose 87 percent.