Not much is known about why we yawn, or if it even serves any useful function. In fact, very little research has been done on the subject because for most of us, yawning is not a problematic behavior. Although not yet fully understood, yawning involves complex interactions between unconscious parts of the brain and the body (1).

Although everyone seems to know WHAT a yawn is, scientists are still debating WHY we yawn. So why is the simple act of yawning so laden with controversy? The unsatisfying truth is that although humans have probably been yawning for as long as we have existed, we still cannot come to a consensus on why we do it. Although there are several theories to explain why we yawn, here are the most common:

~THE PHYSIOLOGICAL THEORY~
The physiological theory encompasses a myriad of biological and brain mechanisms. These theorists believe that our bodies use the act of yawning to either drawn in more oxygen, or remove a build-up of carbon dioxide. Could this be the reason we tend to yawn in groups? Larger groups produce more carbon dioxide, which means our bodies would act to draw in more oxygen and get rid of the excess carbon dioxide (2). But if we yawn expressly to oxygenate our blood, why aren't we yawning continuously while huffing and puffing on mile two on the treadmill? Robert Provine, a neuroscientist from the University of Maryland who is a leading expert on yawning, tested this theory. He discovered that neither supplementing additional oxygen nor decreasing the amount of carbon dioxide in a participant's environment significantly decreased the amount of yawning observed (3).

But the brain itself may provide further insights to the physiological theory of yawning. In the brain, an area called the hypothalamus plays an important role in the physiological mechanisms that may result in a yawn. In particular, the paraventricular nucleus (PVN) of the hypothalamus seems to be the "yawning center" of the brain. Low oxygen levels in the PVN can induce yawning (1). The PVN also contains several chemical messengers that may induce yawning. For example, research shows that yawning occurs when neurotransmitters like dopamine, nitric oxide, glycine, and oxytocin are injected into the hypothalamus of animals (1).

~THE EVOLUTION THEORY~
Some evolutionary biologists believe that our early ancestors may have yawned to intimidate others by showing our teeth (2). The evolutionary theory also entices us to accept that yawns are contagious

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