I’m fighting off a cold and losing, on top of my sleep apnea. So last night many times I woke desperately gasping and panting. My brain fought to make sense of the physical sensation of being unable to breathe by shoehorning in horrible mazes of complex problems I had to solve to get precious air. Also, for extra fun, some of those dreams where you are trying to talk to someone and you simply cannot muscle the words in a line to make sense; too tired, too scattered, utterly unable.
For an articulate person, these dreams are difficult. For a person who plays with orderly systems for fun, these dreams are haunting.
I could see a system of giving wizards spellsinge. For an OSR base: note how many spell levels are cast in a day. Next time you try to sleep, save vs. spells (or whatever is appropriate for the system–Intelligence works too). Every point of margin of failure is 1 spell level slot not available the next day. (Failing by 4 points could be a 4th level spell, two 2nd level spells, a 1 and a 3, etc.) And the dreams claw at the subconscious until a night of clean sleep is managed.
For Crumbling Epoch, I’d make it so every 3 full points of margin of failure is a rest that cannot be regained for the day.