Although not fully understood, yawning appears to be not only a sign of tiredness but also a much more general sign of changing conditions within the body. Studies have shown that we yawn when we are fatigued, as well as when we are awakening, and during other times when the state of alertness is changing.
You are correct in thinking that yawns are catching. Seeing, hearing or thinking about yawning can trigger the event, but there is little understanding of why it is contagious. A number of theories regarding the genesis of yawning have been presented over the years. Some evidence suggests that yawning is a means of communicating changing environmental or internal body conditions to others. If so, then its contagious nature is most likely a means of communication within groups of animals, possibly as a means to synchronize behavior.
If this is the case, yawning in humans is most likely vestigial and an evolutionarily ancient mechanism that has lost its significance. Yawning is a stereotypical reflex characterized by a single deep inhalation with the mouth open and stretching of muscles of the jaw and trunk. It occurs in many animals, including humans, and involves interactions between the unconscious brain and the body, though the mechanism remains unclear.
As for the etiology of yawning, for many years it was thought that yawns served to bring in more air because low oxygen levels were sensed in the lungs. We now know, however, that the lungs do not necessarily sense oxygen levels. Moreover, fetuses yawn in utero even though their lungs arent yet ventilated.
In addition, different regions of the brain control yawning and breathing. Yawning commonly occurs either before or after sleep, which is why it is usually considered a sign of being tired. Yawning also occurs frequently in people who are doing boring or tedious things. There is also a social aspect to yawning.
Yawning appears to be contagious among humans and other animals, and the contagiousness of a yawn is well documented but hardly understood. There is no definitive reason for a yawn yet.
Many proposed theories have surfaced and been studied, and they provide some clues. Yawning is commonly thought to be a sign of sleepiness or boredom, though this is not always the case. While someone who yawns may be tired, the heart rate quickly rises during a yawn.
This increased heart rate suggests yawning can be a sign of alertness rather than sluggishness. People may also yawn when changing physical states as well, such as moving from an area of high pressure to low pressure.
This pressure can build up in the eardrums and may cause the person to yawn to release it. Yawning may be a function of breathing. Yawns may be more likely when the blood needs oxygen.
A yawn causes a big intake of air and a faster heartbeat, which could theoretically mean that it is pumping more oxygen through the body. So a yawn may be simply designed to help clear toxins out of the blood and provide a fresh supply of oxygen.
Yawning may cool the brain. A yawn causes the jaw to stretch out, increasing blood flow in the face and neck. The large inhale and rapid heartbeat caused by the yawn also causes blood and spinal fluid to cycle through the body faster. This whole process may be a way to cool down a brain that has gotten too hot. Some researchers believe the reason humans yawn has more to do with evolution. Before humans communicated vocally, they may have used yawns to convey a message.
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