Most people believe that we yawn simply because we are feeling tired. But in actual fact, scientists do not think that this is the case. In reality no one is completely sure why we yawn, or indeed why it is that yawns are often contagious.
The most commonly recognised theory about the causes of yawning is that a yawn is a reflex movement caused by the bodys respiratory system when there is an imbalance of oxygen and carbon dioxide in the body, particularly a low level of oxygen in the lungs. A yawn will cause a slow exhalation of oxygen after deeply breathing in the gas. This movement is thought to be controlled by the bodys nervous system.
Although scientists do not believe that yawning is caused simply because of tiredness, they do believe that fatigue can contribute. This because when we are tired are breathing is thinner than it is when we are fully functioning, meaning that there is less oxygen travelling around the respiratory system. For the same reason we are thought to yawn when we are bored.
With such a large influx of oxygen coming into the body, the effects of yawn are significant. Increased oxygen will cause the heart rate the increase and thus improve the amount of oxygen travelling around the body. Carbon dioxide levels will decrease, and oxygenated blood will travel around the brain.
Another theory which has been suggested is that yawning has an evolutionary value. This theory deems that yawning is an attempt for early man to show his enemy his sharp teeth and to put them off an attack. This however is not a view which has received much support, particularly as you generally do not yawn when in dangerous circumstances. Another potential suggestion is that yawning is a way to get more air when there may be low oxygen, and the large visual display is used to indicate to others in the area that there may be low oxygen.
This is one explanation as to why so many people find that yawning is contagious, but only if you accept the previous hypothesis. Why is it that a group of friends will find themselves yawning one after another? Science has suggested that contagious yawning is caused by the brain becoming highly suggested from seeing another person yawn. There is very little evidence to support this idea, which leaves yawning still as one of lifes little mysteries.