This is what exactly goes on in your brain when you are in a funny situation or under stress...

Humor, Laughter and the Brain While comedians have spent a great deal of time focusing on humor and laughter, scientists mostly ignored the subject. In recent years, however, several groups of researchers started to scrutinize this form of merriment. Their investigations are shedding light on how the brain processes humor and prompts laughter. Researchers believe that uncovering the brain and body's specific response to positive stimuli like humor and laughter may lead to new therapies. …

…of ACTH. This second hormone travels in the blood to the adrenal glands, which sit on top of each kidney, and instructs them to release a third hormone, cortisol. Together the hormones rally the body systems to prepare you to cope with the situation. You're ready to do battle or flee. Then they help the body return to a normal state. A number of researchers contend that malfunctions in the CRF system often underlie depression.