What Causes Black Outs While Driving?

I recently experienced some sort of a blackout while driving. i was very tired and was sort of weaving over the white lines. I was not too worried as i was not far from my exit. But, I ended up driving about 40 miles beyond where I usually exit the highway. When I sort of "woke up" I realized how far away from my exit that I was but I havwe no recollection at all of driving that far.