Scientists are not really sure why we yawn however, there are four popular theories. Some believe that we yawn when we are tired because it helps to arouse us or prevent us from falling asleep. The reasoning behind this is that every time you yawn, your heart rate increases by as much as 30%. A yawn also causes your lungs to expand to full capacity which allows them to absorb more oxygen. The increased heart rate causes the blood to pick up the extra oxygen at a faster pace, helping you to remain alert. The same physiological response produced by a yawn helps to energize you and get you going upon waking from sleep even though you may not necessarily be tired.
Another theory as to why we yawn when we are tired has to do with an excess of carbon dioxide in the body. It is believed that the exhalation during a yawn expels the carbon dioxide which is responsible for making us feel tired. In this way the carbon dioxide theory is very similar to the one stated above. However, experiments designed to test this theory had participants breath high concentrations of carbon dioxide. The studies showed that higher exposure to carbon dioxide did not cause people to yawn more than they usually do.
A third theory involves evolution. Some researchers speculate that our ancestors would yawn as a form of communication. A yawn would signify members of a group to change their activity. One of these activities could very well be going to sleep. This would not only explain why people yawn when they are tired but also why yawning is contagious. In fact yawning is so suggestive that you can easily start a chain of yawns by simply feigning one. Even reading or thinking about the word yawn may be making you yawn right now.
Finally yawning is believed to simply be a way of stretching the muscles in the jaw and neck before and after sleep just like you would stretch any other part of the body. Stretching is important both before and after sleep to help your body to relax, release tension and maintain proper alignment. This means that yawning when you are tired helps to prepare your body for sleep.
These theories all seem reasonable, but they are still just theories. No one really knows for certain why people yawn. There hasn’t been much research on the subject probably because people have been yawning all their lives, even before birth, and most of the time it isn’t bothersome apart from when someone is expecting you to pay attention to them!