Yawning is a common sign of tiredness, though people yawn for other reasons. The causes of yawning are not well understood (See wikipedia).
There is no direct mention of yawning in the Bible (Ps 5 : 9 is sometimes mentioned, but it’s clear that “their throat is an open grave” is not about yawning).
The ground “yawned” in a particularly dramatic way for Korah, Dathan, and Abiram:
And the earth opened its mouth and swallowed them up, with their households and all the people who belonged to Korah and all their goods. (Num 16:32)
But this is more about swallowing than yawning (and you can’t do both at once).
Yawning is often a sign of sleepiness, and the Bible has much to say about that. For example:
A little sleep, a little slumber, a little folding of the hands to rest, and poverty will come upon you like a robber, and want like an armed man. (Pro 24:33-34)
Correct me if I’m wrong, but I think your understanding of the Proverbs 23 passage is that “a little sleep, …” is talking about being tired because you don’t have enough sleep. I’m not sure this is right. Rather, I would understand this verse as condemning additional little sleeps at inappropriate times. For example, if you have work to do in the middle of the day, and you decide you’ll just have a little shut eye, and then you keep doing it, then it is quite easy to see how you could come into poverty because you just don’t get things done.
I agree, the passage is about procrastination. That’s why it says “poverty will come upon you like a robber, and want like an armed man.” the passage isn’t warning us against sleep it’s warning us against finding “petty” reasons to put off work that needs to be done.
Well no, I think we agree about what it means. But why have a sleep other than you are prepared to give into sleepiness? If you weren’t sleepy, then you’d just be folding the hands?