When I was a kid, my dad (a physicist) would often read stories to me and my brother. He would sometimes fall asleep while reading, and we could tell when that was coming because suddenly our children's story would stop making sense and get filled with all these big physics terms.
Somewhat related, but when I was sleep deprived and falling asleep during a general relativity lecture in college, I caught myself reasoning through the series of equations written by the professor on the board, and agreeing as to their validity with the explanation in my mind being that my parents are in a foreign country. My brain was convinced it had logically checked the progression and that it made sense, but it was based on this irrelevant fact. The experience always makes me wonder what falsehoods we individually or collectively use to convince ourselves of things being true.
Oh yeah, I have been sleep deprived so much that the things that I said made no sense. I still formulated sentences but they did not have any meaning. In fact, I noticed this myself and I was like "fuck, what I just said made no sense". This happened a few times. It is a pretty interesting experience.
I had the same experience after almost 4+ days going without sleep. A friend came to check up on me after I fell asleep, I woke up and started telling him a whole story that made no sense, but I said it with such importance that for a week he was asking me to explain to him what I meant...which I don't remember exactly, but it was important.
So, sleep deprivation leads to temporary schizophrenia? Who could have foreseen this?
Sleep scientists and drill instructors!
This sometimes happens to me when I get a migraine.
A sentence can be coherent in the formulaic sense, but complete nonsense as far as words. I immediately notice that it's incorrect, but I don't have the ability to fix it at that moment.
That is an amazing sleep indicator: once the rabbit starts discussing thermodynamics, dad has left the building