Unfortunately, humor research suffers from lack of funding. However, there is an occasional scholar who helps us better understand what’s happening to our human physiology when we laugh, think positively, or shift perspective from "half empty" to "half full" thinking.

Probably the most controversial "finding" surrounding medical studies involving mirthful laughter is whether or not the stimulation of one of the body’s natural painkillers, known as "endorphins," is actually triggered. Most people don’t really care – as long as they have experienced the phenomenon of pain reduction – what, exactly, has caused the reduction.
However, because of the controversy that exists among scholars, you won’t hear Shirley professing the value of endorphin release during mirthful laughter. You will hear her provide personal testimony, however, with the acknowledgement that "some powerful painkiller is being activated during these experiences, and medical science just hasn’t caught up to the human understanding of what that means."

On a more positive note, there is more-conclusive evidence that mirthful laughter does, in fact, stimulate the immune system. The studies that resulted in this finding were significant because, prior to this, humor scholars had only been able to claim that mirthful laughter reduced the inhibitors to the immune system. In that way, the laughter merely served to put inhibitors at bay so the immune system could do its intended work of fighting infection and disease.
These more recent studies, however, demonstrated not just an inhibitor-suppressant effect, but also demonstrated how, in fact, the immune system was strengthened or made even more effective following a mirthful laughter experience.

Evidence exists that tears of emotions – laughter and sorrow (or grief) – are chemically different than those tears we shed when we get something in our eye, or when we’re peeling onions. Without going through the entire medical explanation (which Shirley understands when it’s described by medical specialists who help explain these phenomena, but which she can’t possible reiterate in these few pages), Shirley summarizes the process by explaining that, the chemical composition of tears of emotions are consistent with the body’s blood chemistry when it (the body) is in a reduced-stress state.
In other words, emotional tears help the body regain a balanced stress level, which in turn allows the systems of the body (cardio-vascular, endocrine, immune, muscular, etc.) to more efficiently be able to perform their respective duties to maintain the body’s state of wellness.

Especially in the case of mirthful laughter, the cardio-vascular and respiratory systems experience very deliberate exercise. For example, the heart rate is increased, and respiration becomes more rapid, causing a profound process of air exchange – exhaling of carbon dioxide, with replacement by oxygenated air which, in turn, feeds the blood throughout the entire capillary system. As a result, body temperature often rises (similar to the effects of exercise) and oxygenated blood feeds the entire body, including the brain (a very good thing for thinking and learning).

Mirthful laughter exercises large and small muscles, including facial, thoracic, abdominal and pelvic, in particular. This activity creates benefits very similar to the alternating contracting and relaxing of muscles during traditional exercise. This action allows fibers and tissues that make up the muscles to efficiently and effectively access the aforementioned oxygenated blood, improving the health of those muscles.
Dr. William Fry, one of the world’s leading medical researchers in humor, has stated that he believes that eventually researchers will have scientific evidence that humor and/or one of its resultant human reactions, mirthful laughter, favorably impact virtually every system in the human body. To date, he says, that remains an exploration into largely uncharted territory, as medical scientists are still discovering new systems not formerly known!