Everybody yawns — from unborn babies to the oldest great-grandparent. Animals
do it, too. But why, exactly, do people and animals yawn? No one knows for
sure. But there are many theories (ideas) about why people yawn.
One is that when we are bored or tired, we just don't breathe as deeply as
we usually do. As this theory goes, our bodies take in less oxygen because our breathing
has slowed. Therefore, yawning helps us bring more oxygen
into the blood and move more carbon dioxide out of the blood.
Yawning, then, would be an involuntary reflex (something we can't really control)
to help us control our oxygen and carbon dioxide levels. Sounds good, but other studies
have shown that breathing more oxygen does not decrease yawning. Likewise, breathing
more carbon dioxide does not increase yawning. Hmmm. Now what?
Another theory is that yawning stretches the lungs
and lung tissue. Stretching and yawning may be a way to flex muscles and joints, increase
heart rate, and feel more awake.
Other people believe that yawning is a protective reflex to redistribute the oil-like
substance called surfactant (say: sur-FAK-tint) that helps keep lungs
lubricated inside and keeps them from collapsing. So, if we didn't yawn, according
to this theory, taking a deep breath would become harder and harder — and that
would not be good!
But there is one idea about yawning that everyone knows to be true. It seems contagious.
If you yawn in class, you'll probably notice a few other people will start yawning,
too. Even thinking about yawning can get you yawning. How many times have you yawned
while reading this article? We hope not many!