Once I didn't sleep for three nights--not by choice, but just because I couldn't--and then on the fourth night I started to have what they call "microsleeps." I would fall asleep for about 15 seconds, and each time it would be a brief flash of a horrific nightmare, worse than my usual nightmares, and then I would jolt awake. Every few minutes this would happen, I'd fall asleep for 15 seconds, and the nightmares kept getting worse. I couldn't control it at all. I still couldn't just sleep straight through for more than 15 seconds, and I also couldn't stay awake for more than a few minutes. I ended up having to go to the hospital and get medicated with benzos so I could sleep.
The microsleeps happen because your body needs sleep and will eventually get it any way it can. I'm not telling you this story to scare you--I think this kind of thing is pretty rare, and that if you're actively choosing not to sleep, once you take it really far, you'll probably just reach a point where you're too tired to function and then you'll just sleep solidly, but I would recommend that you don't wait until your body decides for you when you will sleep.
Another thing to keep in mind is that if you drive, they say that driving while very sleep-deprived is just as dangerous as driving drunk.
Like joeylittle said, a sleep study is a good idea. I had one before and it showed some sleep disorders. It is not very pleasant, because you have to sleep covered with wires, and every time you want to get up you have to call a nurse to unhook you, but it will help them treat you. And not all of the treatments are meds, so it's not like they're just going to dope you up afterwards. When I did it, it was in what looked like a hotel room with a big bed and a big TV, instead of something like a hospital room, so that was nice.
