Both falling asleep and laughing have to do with the realization of something being wrong I think.

With humor we realize we recognize some wrong we've totally missed and have a moment of giving up, which is to say a moment of thinking it's impossible to keep track of everything such that nothing will catch us by surprise and thus we 'give up' for a moment and all the stress falls off of us. Euphoria ensues for a moment and we laugh.

With falling asleep, there is a moment when our thoughts go wrong. Some mistake is made in our thinking that makes our thoughts nonsensical. Or perhaps makes it appear that our thinking is simply irrelevant to reality and the stresses we face within it? Some mistake is made and... it's subconsciously perceived...? And stress falls away, and sleep is possible. Or more likely simply because our thoughts are irrelevant, there is just no stress to attach to them, which leads to sleep.

Which comes first it could be asked? The wrong thought? Or the falling asleep? It could be said the falling asleep causes the wrong thought. I suspect not though. I suspect the wrong thought is slightly ahead of the falling asleep. Very, very slightly ahead. Slight enough the wrong thought doesn't stay in our memory. Because it has quickly triggered our falling asleep, during which our memory fails.

I suspect simply being tired manages to eventually trigger a wrong thought. Which can then trigger a sort of euphoric feeling which means falling asleep.

Not always of course. The wrong thought must place life and its stresses in a state of seeming irrelevance and/or absurdity. From the position of life's stress being irrelevant, stress is now gone and you're free to go away. You can sleep.

For humor the wrongness is very much tied into life's stresses.
For sleep the wrongness must instead simply make life's stresses seem nonsensical, absurd, etc.

Why don't people regularly start laughing while trying to fall asleep?

Humor wrongs are unexpected wrongs that mean we've personally failed, our personal ability to predict the future, etc, to see 'threats' to ourselves coming. (So we give up, shrug all the stress off..., quit trying for a moment to predict all threats...)

Sleep-triggering-wrongs destroy the real world. There is still the "stress" of a world. It's just an entirely different one from the real world. This world, the real world, is gone. And that means generally a lot less stress than you'd normally feel, excepting when you laugh, during which moment there's no stress at all.

And I'm using this word "stress" differently than the norm. Except for the moment you laugh, you are ALWAYS feeling some level of stress.

Perhaps one can purposely think absurd, strange, irrelevant thoughts in order to help one sleep?

A banana came out of Glick's nose. Came. Slowly. Like a slug. Nostril stretched wife. Not crushing the banana. It spun.

Twirled.

As he stood with his head tilted back.

Back?

The world tilted. He stood straight up and down.

Banana Glick. The man of many bananas. They peeked out of his pockets.

"Have a banana?"

Like inch worms they crawl across the floor. Quite slowly. You can easily escape. But let's wait a moment.

How long can we wait?

How far does each stride take them?

Are they capable of different length strides?

Are they mean bananas?

Smile at the bananas.

Excellent banana. Above average circumference. No icky black spots.

Why's it in your nose Glicky?

"Tis an issue of purity."

Unfathomable depths. So many bananii.

Upon the table. There it sat. One fourth a banana. 15 inches thick. The peel of course, thinner relative to the rest of the banana than would be the case in a normal sized banana.

Crawl inside the banana Kathy.

In she peers with a look of wonder.

Further.

"Ah, now I can see forever!"