The International Journal of Psychiatry in Medicine says, "A highly significant increase in survival was due to the psychological variables of block three [quality of life and sense of humor] (p < .001) essentially accounted for by sense of humor (p < .005). Those who scored above the median in sense of humor increased their odds for survival by on average 31%. Conclusions: Sense of humor appeared to mediate better coping and, therefore, protected against detrimental effects of disease-related stressors upon survival."
This finding is in line with the notions that stress weakens the immune system and humor can reduce stress.
Researchers are using the idea that stress reduces blood flow and laughter increases blood flow. A preliminary study by Michael Miller, M.D., and others (all from the University of Maryland School of Medicine, Baltimore) used violent and comedic movie scenes with twenty patients. In 2005 Miller reported that "average blood flow increased twenty two percent during laughter, and decreased 35 percent during mental stress." He said we still need to exercise regularly, but 15 minutes of laughter on a daily basis is probably good for the vascular system.
Bottom line, laughter can improve your health.