engaging article on Madan Kataria, "The Laughing Guru." Research in the field of psychoneuroimmunology has by now provided abundant evident that the opposite of laughter or light-heartedness - depression, stress, fear, or social isolation - can diminish health and suppress the immune system. Kataria's international Laughter Yoga movement is based on the premise that laughter boosts health and the immune system. Norman Cousins, who was editor of Saturday Review, wrote an influential book "Anatomy of an Illness" that described how genuine belly laughter lessened his pain from a joint disease, and several studies have shown that laughter therapy lessens pain in cancer patients. Other work has suggested that laughter may cause transient decreases in cortisol (stress hormone) and increases in endorphin (analgesic) levels, but Robert Provine and others have carried out exhaustive scientific reviews on laughter, humor, and health that conclude that there is not enough evidence of conclude much of anything, other perhaps than laughter can briefly limit physical pain.
I'm inclined to believe that there might be more to it, given numerous example of our embodied cognition. An instruction to contract the facial muscles that form a smile (with no instructions on accompanying feelings) causes slight enhancement of left/right frontal brain activity associated with more positive affect (elevation of mood), and small Botox injections that temporarily immobilize parts of the face can limit one's ability to express or feel emotions. These effects are transient, but it seems plausible to me that repetition of mechanical smiling exercises might enhance mood for extended periods of time, just as repeated meditation practice can decrease brain noise.