"I don't see someone who really enjoys campaigning the way he does," Conway said.

"He keeps such a crazy, difficult pace for a man his age that it's very difficult for the younger staffers, of which I am not one, to keep up."

Conway said Trump is constantly reading and thinking. Conway said Trump is constantly reading and thinking.

"I confess I don't know when he sleeps," she said.

Trump has continued to criticize Clinton's "stamina," including during a speech Wednesday night in Jackson, Miss.

"I think stamina is different from health," Conway said.