I had one stretch of 46 hours without sleep that surprisingly was not too difficult. I was mostly numb, and my memory of it is a little faded. I had plenty of sleep going into it, though, so it was tolerable.
My worst stretch was during my "hell semester". I stayed up until 3am Wed. morning working on a project, and then got up for an 8am class. I didn't sleep that night because I was working on a research paper. I made it to all of my classes the next day, and spent most of the evening studying for a Chemistry final on Friday. I finally took a nap from midnight until 3am, and then resumed studying. I stayed up until 1:30, until I took one shot of whiskey. I babbled incoherently for about 20 minutes, passed out on the couch, and woke up at 3pm Saturday afternoon.