Explaining Body Reflexes

By CBSNews.com staff CBSNews.com staff

June 1, 1999 / 4:59 PM EDT / CBS

Have you ever wondered why you yawn or why yawning is contagious?

In the CBS This Morning series Body Tricks, Health Contributor Dr. Dave Hnida of CBS Station KCNC-TV in Denver, Colo., is back with some more fascinating explanations for things our bodies do.

Why do fingers wrinkle up after being in water?
Skin absorbs water and the skin on your hands and feet is quite thick so it absorbs about 6 to 10 times its weight in water rather like a paper towel.
At the end of your limbs there is nowhere for the water to go so it folds or buckles the skin into ridges and furrows giving it that wrinkly look.
Why do we yawn?
Yawning makes us more alert but why and to what purpose we don't really know. It may be that yawning opens up the blood vessels of the brain. It may just give our bodies something to do when we're tired or bored and waiting to go on to something else.
Researchers have found that athletes yawn before a competition, violinists before a concert, paratroopers before a jump. And maybe we yawn before we go to bed it's just fighting to stay aroused when your body is telling you it wants to sleep.
But it's probably not because we need more oxygen in our lungs (which is the conventional wisdom.) Fetuses also yawn and they don't use their lungs.
Why are yawns contagious?
It's not just humans but it also happens to lots of animals that live in groups like monkeys and lions. One yawns and the others follow. One guess is that we are hardwired to respond to a yawning face, as yawning seems to play a role in synchronizing group behavior.
It's the power of suggestion. One person yawns, then the others and everyone is now aware of each other and on the same page, so to speak.
What about moving in our sleep?
Have you ever started to fall asleep when all of a sudden your body jerks. It feels like "8" on the Richter scale but it's something harmless called a Sleep Start.
Sleep starts happen when the part of the brain that controls muscle tone decides to have one more good contraction before slowing down for the night.
Why do we close our eyes when we sneeze?
Sneezing is a somewhat violent reflex that causes your chest and throat to contract which forces a burst of air to come out through the nose. This would cause pressure behind our eyeballs as well which could rupture some of the blood vessels.
Your eyes probably wouldn't blow out if you didn't close them but they could be damaged. So to protect them, the muscles around the eyeballs contract when eyelids shut to form a solid barrier.

Click here for Part One of this series.