In the last few decades, researchers have studied laughter's effects on the body and turned up some potentially interesting information on how it affects us:
Blood flow. Researchers at the University of Maryland studied the effects on blood vessels when people were shown either comedies or dramas. After the screening, the blood vessels of the group who watched the comedy behaved normally -- expanding and contracting easily. But the blood vessels in people who watched the drama tended to tense up, restricting blood flow.
Immune response. Increased stress is associated with decreased immune system response, says Provine. Some studies have shown that the ability to use humor may raise the level of infection-fighting antibodies in the body and boost the levels of immune cells, as well.
Blood sugar levels. One study of 19 people with diabetes looked at the effects of laughter on blood sugar levels. After eating, the group attended a tedious lecture. On the next day, the group ate the same meal and then watched a comedy. After the comedy, the group had lower blood sugar levels than they did after the lecture.
Relaxation and sleep. The focus on the benefits of laughter really began with Norman Cousin's memoir, Anatomy of an Illness. Cousins, who was diagnosed with ankylosing spondylitis, a painful spine condition, found that a diet of comedies, like Marx Brothers films and episodes of Candid Camera, helped him feel better. He said that ten minutes of laughter allowed him two hours of pain-free sleep.
source : webmd.com