Insanity. Studies have been done on this - I'm sorry I can't be more specific as I read the studies some time ago and can't remember where I read them. The studies have shown that after 7 days, a person is out of their mind. (yes - that's a technical term - haha.)
Actually, sleep deprivation is a "tool" used in "interrogation" and torture.
From personal experience with insomnia, I can say that I have experienced disturbing visual hallucinations - although I was well aware they were not real.