I'm less concerned about his cognitive faculties, though I definitely understand your concerns. Apparently, he was suffering from a gnarly cold last night, and he looked both tired and unwell (though he did better as the evening progressed).
Apparently he's feeling much better today, because he just finished up a rally and looked like his usual self again. Honestly, kind of fired up, in fact.
I think going forward with the debate was a mistake. He should have just rescheduled, and admitted he'd caught a cold. Yes, the right-wing would turn that into "he's on death's door," but that would have petered out, while the soundbites from this debate will have legs for a while.