Nigel,
You raise some interesting points.
I too seem to have reduced symptoms in the middle of the night, although I'm 12 hours or so from my last dose. I can put off my morning dose for hours and still perform pretty well, but once I take a pill I quickly deteriorate until its benefits kick in. Interestingly, when this wears off I'm much worse than I was during the night even though the last dose was much closer.
I think that your neurologist is right that it's to do with your remaining dopamine production. I think, however, that the real answer is more nuanced.
Based on a finger tapping argument I estimate that 60% of my dopamine is endogenous.
http://neurotalk.psychcentral.com/thread183360-2.html
See Post 17.
John