Why is yawning contagious?

"Yawning - an involuntary act that may be brought on by tiredness or boredom. The mouth opens wide and admits a slow, deep breath, followed by an equally long expulsion of carbon dioxide. The heart rate speeds up a little and the eyes may start to water " source: Reader's Digest Why In The World

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The scene: A crowded room in the late afternoon. Suddenly, one person yawns. Before long, others followed and the one who started yawning was accused of having started the mass of boredom and tiredness.Most of us have had an experienced like that. Many researchers dispute that yawning is contagious. One likely explanation to that is this: when one yawns followed by another, it then becomes the subject of comment and impresses the memory. If it happens more frequently than enough, it is believed, without statistical basis, that one yawn has induced another.

Another theory to explain this contagious yawn is that the yawns come when we are all feeling drowsy. If we are together in a stuffy room, crowded, we are all likely to get more oxygen; and if it is in the late afternoon or evening, most of us are tired and yet so keen to get to bed. One friend's yawn may suggest to others that they should do the same. If we are then yawning out of boredom, our subconscious mind most probably is giving us a hint that we could have the meeting to come to an end.

The purpose of yawning is unknown. However theories suggests that yawning is induced by a high level of carbon dioxide in the blood.

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At the time of yawning, by taking a deep breath and exhaling, we are able to increase the blood's oxygen, livening ourselves up.Have you noticed a sudden shift, from sleepiness to attentiveness when you suddenly yawn?

Reference: Reader's Digest Why In The World