Oh, I believe he was indeed tired. In fact, he actually looked drugged. I was wondering if maybe he was taking pain killers for the recent operation. He did indeed look drugged (legally). I'm not trying to imply he's on drugs illegally. I know he recently had an operation and still in pain because of it.
That being said, what really hurt him was the "no heart" statement. And I feel (like most) that the statement came from his heart and not from being tired.
In all 3 debates he looked physically uncomfortable, especially after standing for awhile. I know from my own experience that standing is harder on the back than walking.