Why do we yawn and why is it contagious?

The Conversation: Consider the scenario. You’re driving on a long, straight stretch of country highway at about 2pm on a sunny afternoon, and you’re desperately keen to reach your destination. You’re trying to stay alert and attentive, but sleep pressure is building up. In response you yawn, sit up straighter in your seat, possibly fidget around a little and engage in other mannerisms that may increase your level of arousal. Is this the purpose of yawning? Yawning is generally triggered by several things, including tiredness, fever, stress, drugs, social and other psychological cues. These are generally well documented and vary between individuals.
The question of why we yawn evokes a surprising amount of controversy for what is a relatively minor field of study. We don’t have evidence that can point us to the exact purpose of yawning.

Yawning

Source: Olivier Walusinski MD. baillement.com  2014/05/08

Yawning is a stereotyped behaviour present in most mammals from rodents to humans and has been described since antiquity. Hippocrates considered yawning to be an exhaustion of the fumes preceding fever. Modern medicine did not pay much attention to it until the 1980s, when, with advances in neuropharmacology, yawning proved to be a valuable tool for the assessing dopaminergic activity and the pharmacological properties of new drugs. However, its precise role in human physiology is still unknown and its mechanisms remain unclear.