Studies by Dr. William Fry, Jr: 20 seconds of laughter can double the heart rate for 3-5 minutes. This effect is equal to 3 minutes of strenuous rowing. (Cousins, Norman, MD (honorary). Head First.p 130-138. NY:Penguin Books, 1989.)
Studies by William Fry, MD, psychiatrist at Stanford Medical School: 20 seconds of laughter is the cardiovascular equivalent of 3 minutes of hard rowing, and can double the heart rate for 3-5 minutes. Provides benefits similar to those of jogging. (Hafen, Brent Q., et al. Mind/Body Health. p 550-560. MA:Simon & Schuster, 1996.)
Refer to Laughter – Humor and the Brain for additional information.