Yawn
Yawning
noun
When you lack stretching, your body has to get that energy out somehow, so you end up yawning.
あくび
The reading for this is an exception. Here's a mnemonic to help you:
You've never seen more yawning than when you performed acupuncture (あく) on a bee (び). It was so relaxed from being the one poked for once that it just couldn't stop yawning.
退屈すぎて欠伸が出てきた。
It was too boring and I yawned.