WHY DO WE YAWN ? WHAT IS THE REASON BEHIND IT ?

There are numerous theories on why we yawn and scientists have yet to come to any consensus. One of the first theories for yawning can be traced back to Hippocrates, the father of medicine, who hypothesized that yawning precedes a fever and is a way to remove bad air from the lungs. Based on modern evidence, however, it seems unlikely that yawning serves as a function of the respiratory system.

By the 17th and 18th centuries scientists were challenging the Hippocratic theories of yawning. These new theories focused on the circulatory system, suggesting that yawning causes an increase in blood pressure, heart rate and oxygen in the blood, which in turn improves motor function and alertness. This might explain why many athletes yawn before playing their respective sports. Yet, current tests have shown that the heart rate, sweating or brain’s electrical activity do not increase after yawning.

ANSWER :- 

Today, scientists continue to research the function(s) of yawning. Pioneer researcher on contagious behavior Dr. Robert Provine suggests that yawning is “associated with the change of a behavioral state- wakefulness to sleep, sleep to wakefulness, boredom to alertness…(Provine, 2005).” And more recent studies have suggested that yawning might be connected to brain temperature (Gallup and Gallup, 2008). When the brain becomes warmer than the homeostatic (stable) temperature, we might yawn to cool the brain. It is theorized that cooler blood from the body floods into the brain and the warm blood circulates out through the jugular vein.

Dr. Andrew Gallup and Omar Tonsi Eldakar (2011) discovered that outside temperature might also affect the amount of yawning as well.  If the outside temperature is warmer than normal, then the organism yawns less frequently.  A possible explanation of this is that because the air outside is useless to the organism it does not need to suck in more oxygen through yawning.  However, other tests showed that the amount of yawning increased when both outside temperature and the temperature of the brain increased.