According to a press release from Lion Brand Yarn, researchers from Harvard have found that knitters can actually lower their heart rate by about 11 beats per minute when knitting. While that study is several years old, the Benson-Henry Institute for Mind Body Medicine still notes that knitting, as well as rhythmic breathing, repetitive prayer, yoga and other meditative movements are quite calming. Doing any of these activities for just 20 minutes decreases metabolism, the rate of breathing and heart rate. Brain waves are also slowed.
Since the Institute says that 60 to 90 percent of doctor visits are for conditions related to stress, regular knitting can be a good way to take care of yourself. So if anyone in your family ever tells you you're knitting to much, tell them you're doing it for the sake of your health.