Why do I yawn when I feel bored or tired?

There are several theories as to why people yawn but no one knows exactly why. Here are a few theories:

• Yawning cools the brain
• Yawning helps to draw more oxygen into the blood and drive carbon dioxide out of the blood
• Yawning stretches the lungs and lung tissues, loosens up your muscles and joints, and wakes your body up

References:
https://kidshealth.org/en/kids/yawn.html
https://www.webmd.com/brain/news/20110923/why-we-yawn#1