According to research conducted at the Harvard Medical School Mind-Body Institute, the repetitive action of knitting and crocheting create a relaxation response. The institute's research shows that a knitter or crocheter's heart rate can drop 11 beats a minute and that blood pressure also drops.
The basic ingredients needed to reduce stress - besides some really great yarn and maybe a silver crochet hook and hand-carved knitting needles - "found to be present in [certain] practices in almost every culture, are the repetition of a sound, word, phrase, prayer or movement, and the passive setting aside of intruding thoughts and returning to the repetition."
The research shows that by using your mind in a certain way to elicit the relaxation response "measurable, predictable and reproducible physiological changes occur that can be useful in countering the unhealthy fight-or-flight or stress response."
Believe me, when you're all wrapped up in some yummy yarn, crocheting a mile a minute or cruising toward the completion of a sweater, fighting and flighting just aren't an option.