Because when people are tired. That is, the body needs more oxygen for aerobic metabolism. And yawning can improve oxygen intake (equivalent to deep breathing) to refresh yourself. When the body is tired, your immune system is the weakest (compared with the normal state). At this time, your respiratory system secretes mucus (but you don't feel it, it varies from person to person), and your nose is more! Because it can help absorb germs that enter the body from the nasal cavity. Kill them at the same time!

That's why!