Even though some teachers might get mad at you for yawning during class, it is not a sign of boredom or disrespect. It is not a sign that you are about to fall asleep or are drifting off into your thoughts. Yawning is a natural function that you cannot help.
Since the false facts you grew up learning about yawning are all wrong, you are probably wondering: Why do we yawn?
What are common myths about yawning?
Some people assume that yawning means you are tired, which is not always the case. There are times when you will yawn late at night, when you are ready to retire to bed, but there will be other times when you yawn in the middle of the day when you are fully awake.
Other people look at yawning as a sign of boredom, which is not true either. Even though you might yawn while you are bored during a lecture, you could also yawn while you are in a good mood and enjoying yourself.
Yawning is not reserved to the times when you are tired and bored. It can occur at any time and there is a debate about why that happens.
In reality, no one is entirely sure why we yawn. It is a mystery that still baffles doctors and scientists.
What is the real reason why we yawn?
Some researchers believe that it has to do with regulating body temperature, that it performs a similar function to sweating.
This theory claims that yawning cools down the brain. Since colder air will do this better than warmer air, it’s more common for people to yawn in the colder months.
Basically, when you yawn, you are stretching your jaw, which increases blood flow in the face, neck, and head. The air that you take in while you’re yawning forces a downward flow of spinal fluid and blood from the brain. The cool air breathed into your mouth cools the fluids.
Another theory has to do with our breathing. When we are exhausted and are not breathing as deeply as we normally do, then yawning helps bring more oxygen to the brain. This can make us more alert.
Why is yawning contagious?
You have probably yawned after seeing someone else yawn before. It is an unconscious behavior that expresses your social competence and empathy.
Yawning after seeing another person yawn has been observed in children as early as one or two years old. It has also been observed in dogs.
There are studies that suggest the closer you are to someone, the higher the chance you will yawn after seeing them yawn. That means that you might not yawn when you spot a stranger doing it, but you probably will yawn right after your best friend does.
Even thinking about yawning can cause you to yawn. You might be tempted to yawn right now.
Even though yawning is completely normal, like anything, it is dangerous if done in excess. If you are yawning excessively, then you should visit a doctor, because you could be suffering from a heart condition or a brain illness. 
Source : https://thoughtcatalog.com/january-nelson/2018/07/why-do-we-yawn/