I suppose it all depends on where you are driving as to whether you have time to relax - I am sure that I used to concentrate less, but these days the roads are always packed and you are either in heavy traffic where anything could happen (for me this is typically motorway or urban) - you can of course relax a little bit when stationary, but I find the relaxation quickly turning into wondering why I am stuck in stationary traffic...
And when the concentration is not directly on the actual act of driving or hazard anticipation, it has been forced into looking for artificial hazards such as speed cameras, unmarked traffic police and talivans, or if you are really lucky - the now oh so rare possibility of overtaking. The looking for speed traps takes place even when going below the limit - it is now really engrained.
And on those nice open country roads where the level of traffic is not high enough to to justify the placement of cameras, then I need to concentrate because that is when I can finally give the turbo some exercise
And yes I do notice that I need to take a break after an hour or so, a few minutes standing by the car is normally enough.
Thinking about it, the need for the higher concentration level didn't come about until the roads were swamped in scameras - and this is why I hate both them and other traffic "calming" techniques; they force me to concentrate a lot harder on things that are not connected with safe driving.