text: yawning occurs due to excess co2 in body but why is it contagious?and why does it induces sleep?   causes of yawning an indication of tiredness, stress, over-work or boredom.  an action indicating psychological decompression after a state of high alert.  a means of expressing powerful emotions like anger, rejection or apathy.  a sign that one is not breathing deeply, and not receiving enough oxygen through the body.  a yawn can express strong anti-social messages, and so in some cultures people try to mute or mask them by placing a concealing hand over the yawning mouth.     a long-standing hypothesis is that yawning is caused by an excess of carbon dioxide and lack of oxygen in the blood. the brain stem detects this and triggers the yawn reflex. the mouth stretches wide and the lungs inhale deeply, bringing oxygen into the lungs and hence to the bloodstream. it is almost certain however, that this hypothesis is not correct. one study documented that this effect does not exist ("yawning" by robert r. provine, pages 532-539, american scientist, november-december 2005, vol 93, no. 6). [1]. a more recent hypothesis is that yawning is used for regulation of body temperature. another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that affect emotions, mood, appetite and other phenomena. these chemicals include serotonin, dopamine, glutamic acid and nitric oxide. as more of these compounds are activated in the brain, the frequency of yawning increases. conversely, a greater presence in the brain of opiate neurotransmitters such as endorphins, reduces the frequency of yawning. patients taking the serotonin reuptake inhibitor paxil (paroxetine hcl) have been observed yawning abnormally often.  another theory is that yawning is similar to stretching. yawning, like stretching, increases blood pressure and heart rate while also flexing many muscles and joints. some have observed that if you try to stifle or prevent a yawn by clenching your jaws shut, the yawn is unsatisfying. as such, the stretching of jaw and face muscles seems to be necessary for a good yawn.    a cockatiel yawningthe yawn reflex is often described as contagious: if one person yawns, this will cause another person to "sympathetically" yawn.[2] the reasons for this are unclear; however, recent research suggests that yawning might be a herd instinct.[3] other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack during a full moon. it signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. it can serve as a warning in displaying large, canine teeth, thus proclaiming, "don't attack while i am sleeping..." the contagion of yawning is interspecific (i.e., try yawning in front of your dog). ironically, yawning in public is generally regarded as impolite in the west, but came into fashion in polite french society for a brief period in the late 18th century. oddly, sometimes sympathetic yawning may be caused by simply looking at a picture of a person or animal yawning, or even seeing the word "yawn".  adelie penguins employ yawning as part of their courtship ritual. penguin couples face off and the males engage in what is described as an "ecstatic display," their beaks open wide and their faces pointed skyward.