Why is yoga supposed to help depression and anxiety?

I've read somewhere that it's supposed to help. So what is it that it does? How does it differ from just stretching and trying to relax?
Answer:
i know my answer is going to be weak. but oh my gosh the first day i've tried yoga it was amazing. i've never been so relaxed before. i've found that since i've stopped it i've gone back to being moody.

i got this from wiki:
"For people with heart problems, studies have shown yoga to help people young and old. Specifically, yoga seems to promote heart health in several ways, including regulating high blood pressure and improving resistance to psychological stress.[1] Yoga also has the potential to buffer against the harmful effects of bodily self-objectification as well as to promote embodiment and well-being[2]
For people with back problems, studies have shown yoga helps physically with strength, flexibility and endurance. Yoga also helps create relaxation through the breathing and meditation techniques employed, which has been shown to help people better proactively manage a chronic pain problem like back pain.[3]"

edit: here http://www.yogajournal.com/newtoyoga/820...

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If you read up on Yoga, it is healthy for body and mind. The different poses are generally for different parts of the person.

Check out Yoga at:

http://www.about.com

Type Yoga in their search box (upper right corner), I think you'll be pleasantly surprised at the wonders of Yoga. :)

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Oh yes definatley, anything positive usually helps. It really helps to relax your mind and call all those racing thoughts from anxiety.

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