We all have nights like that--least I do :0 (that's a yawn!).

Funny thing with me is that there is a sweet spot between getting tired, and becoming incapable. It varies between 1/2 hour and a couple of hours in which everything just seems to click. I can be more productive in that time than throughout the rest of the day. The problem is recognising when I transition from the state of grace to "what a state". It's not always obvious to me at the time, as there are a few indicators around this site.