How is it possible to drive while "asleep" (or while drunk)?

First of all, I have never and would never drink and drive. However, I confess that there have been a very few occasions (one very recently) in which I was absolutely exhausted while driving on the highway, and in my exhausted state, didn't even realize that I was continuously falling asleep at the wheel... I just go numb and jolt awake and realize that I have been just drifted for a few seconds, and this happens for several minutes before I truly become cognizant of the fact that I am actually falling asleep at the wheel and need to get off the road. As terrifying as these events can be, I am fascinated by the science behind this. Obviously when this happens, I am not actually closing my eyes and going to sleep, otherwise the odds that I would still be alive are astronomical. I read that this can happen when exhausted and some parts of the brain shut down, but others are still awake. When this happens I am completely unconscious of what I am doing, and yet maintain control of the vehicle for several moments at a time. I am sure this is a similar sensation to drunk driving? In which you black in and out of consciousness but were more or less still "driving the car" (unless of course you crash). I know it's bad, but what is going on here in the brain?