According to researchers at the Boston University School of Medicine, yoga postures and the deep, controlled breathing associated with yoga might help to ease anxiety and depression. Brain scans of yoga buffs reveal a boost in levels of gamma-aminobutyric (GABA), a neurotransmitter associated with combating anxiety and depression. Apparently, the school of yoga doesn't matter - whether practitioners were trying vinyasa, hatha, or ashtanga didn't make a difference: their GABA levels were still high. Other researchers insist that the yoga group should have been compared to another movement-based group rather than just a sedentary set of subjects, since physical exertion might have more to do with brain changes than yoga itself. However, the researchers who did the study believe that it's the mind-body treatment connection that makes yoga a more viable activity for combating stress and depression.
Interesting study, I know that I do feel better when I am practicing yoga at least once a week vs when I do not get any yoga in. Although getting in any excercise tends to help me deal with the depressive side of bipolar. Yoga helps teach you to breath properly so maybe that also helps me to deal better with added stress. I tend to agree with the other researchers that it was the excercise and not exactly just the yoga.