Most Helpful Girl
Because when someone does it, it reminds you just how tired you actually are... I guess.
The word itself made my yawn a bit just now.1
Most Helpful Guy
Human evolution, it was to signify that the lookout was tired, and the lookout being tired would mean that he/she couldn't sense any danger, so it was safe to sleep. It was contagious so that the message would spread throughout the group.1