I don't have a link, I roll by experience. Most research I use the Internet for is more in depth.
So excuse me for citing experience, but that's all I have so I'll try to convey it best I can.
It's not always light, and I didn't say it was. Sometimes it's heat or humidity, though humidity is usually directly affected by the heat.
You stress your plant by training it and it rewards you with doing what you want. If you stress it too much, it's unhappy, hence the bleaching and drooping.
When the leaves push upward like that and start folding, they are trying to pull more humidity, decrease light on surface area, or trying to transpire more from the heat. Like a dog panting, because plants and dogs don't "sweat". Take the sweating part lightly.
Now, if you have them praying and you leave them like that for a few weeks, maybe less, you'll see the stress. Whether bleaching or just "tired" leaves, which you noticed from the slump. The slump means too much to fast. Praying would have been that sign.
Take that for what it's worth, like I said, just my experience, but if you have a praying plant and back the light off a tad, it won't pray anymore. So it must be a sign in my opinion.