Dr. Lee Berk and Dr. Stanley Tan have been researching laughter and the body's response to it for well over twenty years. They've learned that laughter fine-tunes the hormones, and decreases stress hormones. Laughter hones the immune system, increasing antibodies and triggering T-cells and Natural Killer cells. And they have reason to believe that there is much more to learn about this pleasant pastime.
"In the current study, 14 healthy volunteers were recruited to a three-week study to examine the effects that eustress (mirthful laughter) and distress have on modulating the key hormones that control appetite. During the study, each subject was required to watch one 20-minute video at random that was either upsetting (distress) or humorous (eustress) in nature."
The volunteers who watched the distressing video had no change in appetite. But the volunteers who watched the funny video had an increase in appetite. This was determined by measuring the two hormones leptin and ghrelin.
http://www.eurekalert.org/pub_releases/2010-04/foas-sfb042210.php