For today’s Stream of Consciousness Saturday prompt from Linda G. Hill, we are asked to “write about the first thing that comes to mind when you think of the word ‘yawn,’ when you sit down to write your post.”

I have to admit that when I read that the prompt was “yawn,” I let out a big one. A yawn that is. Probably because it’s almost midnight and I’ve been up since 6:30 this morning.

But then I thought about why we yawn. Everybody yawns. And it’s not just people. Animals do, too. But why?

One theory is that when you’re tired or bored, you don’t breathe as deeply as you do when you’re awake and engaged. As a result, your body takes in less oxygen because your breathing has slowed. Yawning, therefore, helps you bring more oxygen into the blood and move more carbon dioxide out of the blood.

Another theory is that yawning cools the brain. When you start to yawn, powerful stretching of the jaw increases blood flow in the neck, face, and head. The deep intake of breath during a yawn forces a downward flow of spinal fluid and blood from the brain. The cool air breathed into the mouth cools these fluids. Together these processes may act like a radiator, removing warm or hot blood from the brain while introducing cooler blood from the lungs and extremities, thereby cooling the brain’s surfaces.

Yet another reason you may yawn is to wake yourself up. The yawning motion stretches your lungs and their tissues and causes your body to flex its muscles and joints. It may also force blood toward your face and brain to increase alertness.

These are all theories about why we yawn. But one thing is certain. Yawning is contagious. Even thinking about yawning can cause you to yawn. How many of you yawned when you started to think about what to write for this post? Be honest.

Yawning is one of the most contagious, uncontrollable actions a body does. In fact, 60-70% of people find that seeing a person yawn in real life or in a photo — or even reading about yawning — compels them to do the same thing.

Some studies suggest that people who yawn when they see other people yawn are actually unconsciously demonstrating empathy and bonding. According to these studies, the less empathy a person has, he or she is less likely to yawn after seeing someone else yawn.

I don’t know if yawning is caused by the need for more oxygen, the need to cool down a hot brain, or because you see someone else yawn and you empathize with them. All I know is that it’s past my bedtime and I can’t stop yawning.

So good night, sleep tight, and don’t let the bedbugs bite.