Why am feeling yawn.

As in poor sleep, or in hard room we are feeling yawn, thus organism needing in enough oxygen, but why I want to yawn,if somebody near is yawns, does she is contagious?
Yes yawn display of poor oxygen, poor ventilated room, or of boring, as mouth is opening width, breath with widening maximally, as in begin is deeply breathe in, after breathe out, and of this air is lung reneview, and body of cells and brain are getting not enough oxygen. And yawn is contagious and we are doing same,like a near person, and this reflex tied,that person are lived ,like together and with spreader emotions with each other, and the same time gone in bed as with yawn transferred a feeling for tiredness,and an other also yawned as a sign for a sleep begun. And this is interesting,that many of non vertebrae animals are yawn involuntarily,just chimpanzee and peoples are to imitate for a foreign yawn.