One of the few times I've knowingly hallucinated happened in college, when I'd been awake three or four nights at the end of a term.
It was a combination of responsibilities and partying. Our academic workload was unusually rigorous, so that the demands for final papers and presentations were strenuous. There was my work on the school arts & letters magazine. There was a late-night radio show. I was tasked by committee with drafting a particularly arduous faculty evaluation. And I was heavy into demonstrative Jack Daniels as my then-characteristic social pose.
It happened during a break, in an empty dorm lobby. In that tired way I began contemplating the stucco patterns on the face of the cinderblock walls. To my astonishment they began moving, forming themselves into fully-articulated real-life shapes which I knew perfectly well were imaginary, yet which had the full definition of, say, animated cartoons. There was a basketball player leaping and spinning, grabbing rebounds, hanging from the hoop. There was a ballerina gracefully turning, fingers and toes pointed, neck craned like a swan from Swan Lake. I watched with great fascination, head spinning, until fatigue threatened to throw me into full sleep then and there.
Nothing as vivid as that has happened since. But to this day if I find myself fascinated by mundane surfaces, for instance the pattern of islands and clouds in the very-white wall across from my desk here at work, I realize that's a clue I'm not sleeping enough.