Best Answer - Chosen by Voting
It's not used for someone who gets up early. It's used for someone who feels energetic and awake when they've just got up, or early in their day. People often say "I'm not a morning person", which just means "if you want to talk to me when I'm in a better mood, don't speak to me when it's still early". Technically there is no opposite to a morning person. A morning person is a term for people who like to be up early and who are bright and alert, you're either a morning person or you're not. So I'd say there is no opposite. But it's true that a person who comes alive and feels comfortable at night (like me) is a nocturnal person or a night owl.
