Yawning: Why and What Could It Mean?

Decreased arousal: 
You yawn when you are in a less stimulating environment (i.e. you're bored).Although excessive yawning is usually attributed to being sleepy or bored, it may be a symptom of an underlying medical problem. Certain conditions can cause a vasovagal reaction, which results in excessive yawning. During a vasovagal reaction, there's increased activity in the vagus nerve.In addition, different regions of the brain control yawning and breathing. Still, low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus of the brain can induce yawning. Another hypothesis is that we yawn because we are tired or bored.