Why Do People Yawn?

Why do people yawn excessively when they are very tired? People live by inhaling oxygen and exhaling carbon dioxide. After

a long time of Study or work, both body and brain are very tired and, at the same time, more carbon dioxide is produced. If the carbon dioxide is not exhaled in time, it MU harm people's health. Yawning is a kind of deep breathing. It exhales carbon dioxide and inhales oxygen. So yawning is good for your health.