If you don't sleep on your belly (which you shouldn't, anyway, you will suffer from back pain after a few decades — trust me, I went through that :-P ), the breastforms should hold well, even sleeping on your side, and not break/rip, specially when protected inside a good, well-fit bra.
This is an information I got from a representative of
Amoena. They do prosthetic breastforms for women who had to surgically remove their breasts due to breast cancer, and, as such, they truly have a lot of experience with people "constantly wearing" their breastforms. Of course they're very friendly (and discreet!) with the CD community and, as such, I now always shop with them (physically; getting them fitted is something best left to an expert). Obviously I had to ask about sleeping in them. While they don't recommend it — probably because they don't want to get sued if the breastforms do, indeed, break or rip if you are really careless about it — they are well aware of thousands of happy customers who sleep in them all the time without a glitch.
I'm superstitious and seldom sleep in them, though, although I've done so occasionally; I just love the extra weight on top of my chest

But if all else fails, slipping into a well-padded B-cup bra is very pleasant as well (the brain, as you so well noticed, will "fill in the details", "knowing" that something "ought" to be there), and at least I won't worry about any damage to my breastforms...