Current evidence demonstrates that spontaneous laughter is associated with greater reduction in cortisol levels as compared with usual activities, suggesting laughter as a potential adjunctive medical therapy to improve well-being.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10204943/#:~:text=Current%20evidence%20demonstrates%20that%20spontaneous%20laughter%20is%20associated,a%20potential%20adjunctive%20medical%20therapy%20to%20improve%20well-being.

A summary of some of the positive findings from research that shows laughter promotes physical, mental, and emotional health and contributes to health, happiness, and wellbeing.

https://link.springer.com/chapter/10.1007/978-981-99-3493-5_8

The high sense of humor group appraised less stress and reported less current anxiety than a low sense of humor group despite experiencing a similar number of everyday problems in the previous two months.

https://scholar.google.com/scholar?&q=Abel%2C%20M.%20H.%20%282002%29.%20Humor%2C%20stress%2C%20and%20coping%20strategies.%20Humor

and

https://www.degruyter.com/document/doi/10.1515/humr.15.4.365/html

Context • A recent survey of rural Midwestern cancer patients revealed that humor was one of the most frequently used complementary therapies. Psychoneuroimmunology research suggests that, in addition to its established psychological benefits, humor may have physiological effects on immune functioning.

https://digitalcommons.wku.edu/cgi/viewcontent.cgi?article=1008&=&context=nurs_fac_pub&=&sei-redir=1&referer=https%253A%252F%252Fscholar.google.com%252Fscholar%253F%2526q%253DBennett%25252C%252520M.%252520P.%25252C%252520et%252520al.%252520%2525282003%252529.%252520The%252520effect%252520of%252520mirthful%252520laughter%252520on%252520stress%252520and%252520natural%252520killer%252520cell%252520activity.%252520Alternative%252520Therapies%252520in%252520Health%252520and%252520Medicine.#search=%22Bennett%2C%20M.%20P.%2C%20et%20al.%20%282003%29.%20effect%20mirthful%20laughter%20stress%20natural%20killer%20cell%20activity.%20Alternative%20Therapies%20Health%20Medicine.%22

From AATH Member Dr. Le Berk:
The addition of mirthful laughter as a preventive adjunct therapy in diabetes care raises HDL cholesterol levels and lowers inflammatory cytokines, and hs-CRP levels. These modulation effects can contribute to lower CVD risk and fewer MI reoccurrence.

https://faseb.onlinelibrary.wiley.com/doi/abs/10.1096/fasebj.23.1_supplement.990.1

Yes, this was a study. The influence of humor on desirability

https://www.sciencedirect.com/science/article/abs/pii/S1090513805000565?via%3Dihub

Discomfort thresholds increased for subjects in the laughter-inducing condition. Laughter, and not simply distraction, reduces discomfort sensitivity, suggesting that laughter has potential as an intervention strategy for the reduction of clinical discomfort.

https://doi.org/10.1007/BF00846422

Anatomy of an Illness (As Perceived by the Patient) by Dr. Norman Cousins

National Book Award Finalist: The “amazing” New York Times bestseller about the power of
laughter and optimism in fighting serious illness

https://www.degruyter.com/document/doi/10.7312/mona92982-009/html

although laughter forms an important part of human non-verbal communication, it has received rather less attention than it deserves in both the experimental and the observational literatures. Relaxed social (Duchenne) laughter is associated with feelings of wellbeing and heightened affect, a proximate explanation for which might be the release of endorphins.

https://royalsocietypublishing.org/doi/10.1098/rspb.2011.1373

https://doi.org/10.1098/rspb.2011.1373