Almost everyone yawns when they feel sleepy. So do
most animals.But we don't completely understand why people and
animals for that matter, yawn.
It's widely assumed that yawning occurs because we are tired or
bored or because we see someone else doing it, but there isn't any
hard evidence to support these beliefs.
Scientists do not purport to know
all of the biological mechanisms of the yawn, but tend to agree that
a yawn is an involuntary respiratory
reflex, which regulates the carbon dioxide and oxygen levels
in the blood.
Technically, a yawn is the reflex opening of the mouth followed
by the deep inhalation and slow exhalation of oxygen.
The very act of yawning is but one of a number of involuntary
reflexes controlled by the spinal and
nerve centers.
Scientists speculate that the onset of a yawn is triggered either
by fatigue, or by sheer boredom as, at those times, breathing is
shallow, and little oxygen is carried to the lungs by the
oxygen-toting cardiovascular
system.
When one yawns, his or her alertness is heightened, as the sudden
intake of oxygen increases the heart rate, rids the lungs and the
bloodstream of the carbon dioxide buildup, and forces oxygen through
blood vessels in the brain, while restoring normal breathing and
ventilating the lungs.
This quite plausible theory of yawning falls short of explaining many aspects of
yawning. Scientists explain away the "contagious" nature of yawning,
that is when one person's yawn triggers another nearby to yawn, as
due to the power of suggestion, but are at a
loss when attempting to explain why yawning occurs
excessively in patients with lower brainstem damage or with multiple sclerosis.
Other unlocked mysteries include why fetuses in the womb yawn, when it is a well-known fact that
they do not intake oxygen into their lungs until after live birth,
or why individuals with high concentrations of oxygen in their blood
streams yawn.
Until these questions are answered, do not assume that a person
who yawns in your presence is bored with what you are saying, or
suffers from exhaustion. Simply be pleased that he or she is not
bored to death.
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