Question

we yawn when we are sleepy or tired because we require sufficient oxygen, but why don't we get enough oxygen when we are tired or sleepy?

Answer 1

Yawning is reflux that occur during tiredness or before or after sleep.
There are several theories that provide reasons behind yawning. One study state that we yawn when there is an increase amount of carbon dioxide in our blood and so we require sufficient oxygen that can be provided by the action of yawning.
In fact yawning decrease the oxygen intake as compared to the normal respiration. It is reported that yawning occur even when the amount of oxygen is normal in our body.