There are plenty of mysteries about the human body, and yawning is no exception.

It's not particularly flattering — and it's sometimes even seen as impolite — but we all do it. And when the urge to let out a yawn comes on, we often can't help it.

We typically think of yawning as an indication that we're tired or bored, but some people report yawning more frequently when exercising or while singing. So, if it's not just a sign that we're sleepy or bored, why exactly do we yawn?

And then there's the most puzzling part of yawning: It seems to be contagious. In fact, it's so contagious that you don't even have to see another person yawning to yawn yourself. Sometimes, all it takes is just hearing a yawn or thinking about one. You may even be yawning right now.

So how much of yawning is mystery and how much can science actually tell us about yawns?

Why do we yawn?

Experts classify yawns into two types: A yawn that occurs on its own, which experts call spontaneous yawning, and a yawn that occurs after seeing someone else do it, which experts call contagious yawning. (Yep, secret's out of the bag — yawning is indeed contagious.)

But, whether spontaneous or contagious, why do we even do it in the first place?

As it turns out, we don't really know why we yawn.

Some theories, however, are that yawning helps:

• Regulate your brain temperature
• Wake your body up
• Bring more oxygen into your bloodstream
• Keep your lung tissue lubricated
  
  

While some of these theories have been largely debunked, one of the more recent and prominent theories that still holds some weight is that yawning may be a way to regulate brain temperature.

Let's back up for a minute, first.

Your brain operates best when it's running at an optimal temperature. When your brain is running warm, for instance, cognitive performance can suffer. As a result, your body has several ways of responding to changes in temperature, such as sweating or shivering, widening or narrowing your blood vessels, and triggering behaviors that cause you to seek cooler or warmer air (i.e. hitting the pool in the summer or putting on your favorite pair of cozy socks in the winter).

The brain thermoregulation theory suggests that yawning is yet another mechanism to help cool your body down — specifically, to cool your brain down.

A body of research supports this theory. Not only have animal studies shown that yawns are often preceded by rising temperatures in the brain and followed by a reduction in these temperatures, but a correlation between ambient air temperature and the likelihood for people to yawn also exists. The overarching theory is that — given that the air around you is of optimal temperature — yawning can serve to help cool your brain.

If true, this concept would provide physiological relevance to yawning. Cooling your brain via yawning could possibly help ensure optimal cognitive performance. However, there are many skeptics of the brain-cooling theory of yawning.

But whether a physiological reason for yawning exists or not, what experts do agree upon is that yawning is indeed contagious.

Why are yawns contagious?

Similar to the mystery surrounding why we yawn, experts also aren't really sure why yawns are so contagious.

What is known is that spontaneous yawning — for whatever reason it may occur — is an ancient, evolutionarily conserved behavior shared among many vertebrate animals.

Contagious yawning, however, seems to be isolated to just a few animals, including people, non-human primates and domesticated dogs. What's more is that contagious yawning doesn't really appear in human behavior until early childhood. Taken together, experts believe that contagious yawning may be a social communication tool specific to higher-order animals.

In the context of the brain-cooling theory of yawning, perhaps yawning evolved to become contagious as a means to increase the cognitive performance and vigilance of people within a group. While this may sound silly in today's world, it could have been an important collective behavior for our cave-dwelling ancestors who needed to stay alert as a group in order to survive.

However, those who dismiss this physiologically relevant theory of yawning do still believe that the behavior has been conserved throughout evolution because of its social effect.

Specifically, these experts believe that yawning plays a role in social communication — that it's a sign of some internal state of mind. Perhaps yawning did truly help our ancestors say: This experience is unpleasant, but not alarming. (In today's world: I'm bored.)

While it's not clear what yawning truly communicates, some experts believe that the contagious nature may also be a way of showing empathy and matching the emotional states between people. In fact, a recent study shows that we're more likely to experience contagious yawning with our closest acquaintances than we are with strangers.

So, next time you yawn, think about whether you're tired, bored or neither, as well as whether you're yawning on your own or you're yawning because it's on your mind. You may even try to decide for yourself which theory of yawning you believe most.