...Laughter stimulates the left prefrontal cortex, an area of the brain that helps us to feel good and be motivated. That stimulation alleviates anxiety and frustration. There is evidence that laughter can send chemical signals to actively inhibit the firing of nerves in the amygdala, thereby dampening fear. Laughter, then, can help temper negative emotions.
Laurence Gonzales in Deep Survival, page 41.