This seems logical since yawning does bring in more oxygen with a deep breath and the expiration removes more carbon dioxide than the usual breath, but research by putting people in low-oxygen or high- carbon-dioxide environments does not cause yawning.
The short answer is that yawning is normal. It is common and usually is totally benign. However, if there is an increase in yawning that cannot be explained by lack of sleep or some of the other causes mentioned above, then yawning can be a symptom of some disease. The most common medical problems that are associated with increased yawning are sleep deprivation, insomnia, sleep apnea, narcolepsy, and many medicines that cause sleepiness.
There are some other medical diseases that cause yawning including bleeding around the heart, brain tumor, multiple sclerosis, stroke, and even heart attack. In most people yawning is a normal reflex, although poorly understood. However, if you experience excessive yawning for no apparent reason, it is wise to visit your physician and make sure there is nothing going on that is abnormal.
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. Still, low oxygen levels in the paraventricular nucleus PVN of the hypothalamus of the brain can induce yawning. Another hypothesis is that we yawn because we are tired or bored. But this, too, is probably not the case because the PVN also plays a role in penile erection, which is not typically an event associated with boredom.
It does appear that the PVN of the hypothalamus is, among other things, the "yawning center" of the brain. It contains a number of chemical messengers that can induce yawns, including dopamine, glycine, oxytocin and adrenocorticotropic hormone ACTH.
ACTH, for one, surges at night and prior to awakening, and induces yawning and stretching behavior in humans. The process of yawning also appears to require production of nitric oxide by specific neurons in the PVN. Yawning likewise appears to have a feedback component: if you stifle or prevent a yawn, the process is somewhat unsatisfying. Temperature changes in the body are an important part of transitioning between sleep and wakefulness. Research shows brain temperature does fall after yawning —and that yawning happens less often in colder temperatures than warmer ones.
So, less yawning on a chilly winter day than a steamy summer one. Yawning triggers blood flow to the brain and its surrounding areas of the head, neck, and face, which pulls heat from the skull.
Do your eyes ever tear up when you yawn? Transitioning through different states of energy and alertness. Yawning may be a way the body both signals and influences transitions from one state to another. That post-exercise yawn may be a way the body redirects blood flow to the brain from its major muscle groups—and it may also be a way the body cools the brain after heating up through exertion.
Yawning may be a signal of a shift from a relaxed state to an anxious, vigilant one. Social messaging. Yawning is an ancient behavior among humans, other mammals, and most vertebrates. Yawning has long been considered by many scientists to be a form of communication that evolved before humans had spoken language. We may yawn to signal tiredness, boredom, stress, hunger, or any number of messages.
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