Facts
* Average duration of a yawn is about 6 seconds.
* The earliest occurrence is approximately 11 weeks after conception.
* Yawning becomes contagious between the first and second years of life.
* Hypothalamus is directly related to yawning

Researches show that the presence of certain neurotransmitters such as dopamine, excitory amino acids, neuropeptides and nitric oxide increases yawning when it is injected to hypothalamus in the brain. Also, tests have shown that the yawning duration is longer in male than in female. Scientists tried to come up with theories to explain the nature of yawning; however, the reason behind it is not yet fully understood. One of the theories was that the reason why human yawn is because due to lack of oxygen and higher level of carbon dioxide. Carbon dioxide is a by-product of cellular respiration along with water. Since one lacks oxygen, one yawns to take in more oxygen and letting out more of carbon dioxide than normal state of breathing. However, an experiment conducted by Dr. Robert Provine proves the theory wrong. He had 100 students taking a part in the experiment where he had four rooms with different level of oxygen and carbon dioxide. The normal air would have 21% of O2, and 0.03% of CO2.

What Dr. Provine found was that the students who were in rooms that had gas#1, gas #2 and #3 were breathing faster, but the different gas level did not increase the number of yawning. Therefore, he disproved the theory and he came up with two new theories about yawning:

1. Yawning is like stretching; therefore, it relaxes the facial muscles and it reduces the muscle tension after yawning. Also, it increases blood pressure, heart rate and flexes muscles as well as joints. Therefore, this theory supports why it feels unsatisfactory when one yawns with his/her mouth closed.
2. The reason why yawning is contagious is because in one time in evolution, it was used as a way to signal each other; therefore, when one yawns the others follow. The reason why it is still contagious is because of the leftover response that human inherited from the ancestors.