What you say about self certification of your "actual" ability to fly, rather than your "legal" ability is hitting it on the nose.
When you sign the doc at the flight schools office certifying you are not ill, not under drugs, harrassed by wife, whatever, it is done momentarily simply as a cover the flight schools ass in the event something happens. The FAA is also covering their ass with their certification.
After you fracture every bone in your body after an accident, and the FAA goes to your doc and examines your medical records, showing you have dementia, bad heart, bad lungs, bad blood, bad children, you can simply say, he never told you that, you had your ipod plugged in your ears.
A good lawyer can spin that for you.
Now, as for yourself, before going to fly, as you have said, you have to "self assess".
Only you know your own abilities and you have to be honest with yourself, something pretty hard to do for most people.
With my big 20 hours in log book, there were times after the ground check of plane, the runup, and all the other info flying into my head, I wanted to walk out of the plane go home and go to sleep. I was in no mood to fly, but that usually went away when I got up there and forgot I was tired.
I have not yet soloed. And even if and when the CFI gives me the green light, will I be mentally up to it? Will I be tired and groggy after going through the whole routine?
I habitually get up at 3 am each morning, so by the time the afternoon rolls around, I need a nap, or snooze. I try hard to get the lesson at 9 am, but sometimes they delay and fart around at flight school and I don't get going to severl hours later.
I guess if he threatened to deprive me of my bagel each morning, with lox and cream cheese, I would do it. But I guess all newbies have this feeling.
If he told me to go solo, and I did feel a bit wiped out from the lesson or haven't slept well the night before and was punchy, I would decline, of course. CFI knows my abilities through his experience, I know myself through my own experience.
I know I need to eat breakfast, I need to fly early, I need to drink some coffee and need to take a pit stop before entering the plane. I need to be reasonably alert, and to solo, need to do it early. Also need to be calm, that is my strength when flying, I move things very slowly and deliberately, perhaps too slowly for some instructors as I take my time lifting off on roll. I take my sweet ass time when operating the plane, perhaps some CFI's are up tight or in a hurry.
I guess the question here would be, according to reports of accidents by NSTB, how many were due to pilot fatigue or illness?
Perhaps the medical exam for PPL should be looking at other factors instead of placing emphasis on conditions that may very well not affect a person's ability to fly safely.
yozz