The short answer is that no one really knows.
The long answer is that no one really knows, but there are plenty of interesting theories:
1. The idea that we yawn to get rid of carbon dioxide and take in more oxygen has been disproved by research, but persists as the "common wisdom" answer. According to this theory, people breathe more slowly when they're bored or tired and less oxygen gets to the lungs. As CO2 builds up in the blood, the brain reflexively prompts a deep,... READ ON