Yawning & one tired?
Answer:
Usually we yawn when we are tired because the brain is lacking oxygen, and so make up for it by yawning and taking in extra oxygen. Sometimes consistent medications can create you to yawn for no reason. I am a chronic yawner myself because of that. So sometimes even when you are awake, your brain or body may be compensating for a want of oxygen at that time and so you yawn.
Yawning has intensely little to do with person tired. It is actually a path for your body to regulate oxygen levels.
We yawn when our body does not hold enough oxygen.
Breath more!
Yawn smaller number!
Just like the above posters said, yawning happen when the brain is lacking oxygen and you purloin in extra O2. When we're tired, we fluently slow down our respiration which may be why our brain is lacking oxygen contained by the first place...