[Or lack of sleep. He did a wonderful rally in NH without any sleep this morning]
So he has a stamina issue? He should ask Trump what his secret is.
Trump isn't high energy - he's just got the manners of a dry drunk - i.e. blind aggression is what Trump passes off for energy. In his low energy state, he can't answer questions - he throws a drunk tantrum and avoids them by accusing his questioner of bias.