In spite of popular misconception, you don't yawn because you're tired.You yawn because your body has built up too much carbon dioxide and needs to release it.
When you're in an inactive state (sitting in class or something like that) your body does not have much opportunity to expel excess carbon dioxide. Excess carbon dioxide in the bloodstream is dangerous, so your body responds by yawning and expelling more carbon dioxide that you would normally do by breathing.
When exercising, you breathe heavily, so it's easy to expel excess carbon dioxide. It only becomes an issue when you're resting... whether it's because you're tired or because you're bored and sitting still.
Yawning does seem to be contagious, but that seems to be a psychosomatic effect. Basically, you see someone yawn, and you feel compelled to yawn. I don't know that there's much more to that. Some things in biology are just quirky that way, or we just don't understand them well enough yet.
Source: answers.yahoo.com
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