Conclusions

Fatigue leads to a deterioration of driving performance, manifesting itself in slower reaction time, diminished steering performance, lesser ability to keep distance to the car in front, and increased tendency to mentally withdraw from the driving task.
Drivers try to compensate for the influence of fatigue, for instance by either increasing the task demands (e.g. driving faster) or lowering them (e.g. increasing the safety margins by slowing down or using larger following distances,). These are compensatory strategies that drivers select to ward off the mental and physiological effects of fatigue.
When drivers are well rested and when they take enough breaks during driving, they can drive for a long time without performance decrement.
One reason compensatory strategies are bound to fail is that during fatigue persons lose the ability to appraise own driving performance. This is especially the case for driving in monotonous road environments.
A separate, different phenomenon is "Driving without awareness" which can be induced by monotony of driving environment. Driving without awareness can occur without the driver being particularly tired.