Sometimes people are invisible but they have arrows stuck on them, which is why I can see them some of the time... Just thought I'd point that out :p it isn't always hacks
That would be because when you're exercising you're breathing pretty heavily already. I looked it up, also, and it said that it is to bring more oxygen into the blood.
I think yawning is when the body needs more oxygen... It's associated with sleep because when you don't get enough you can fall asleep? Not entirely sure, but yeah xD