Chatting with a girl and saying good night and going to bed early, she didn't reply. The next day, she said you too, take good care of yourself. What does she mean?

From your description, we can see that the girl is still polite. There are three possibilities for him to do so. 1 The first possibility is that this girl is really asleep, so you say good night to him. He didn't see it when he went to bed early, so he will say such a thing the next day. The second possibility is that this girl is not asleep, and she just doesn't want to reply to you. However, the next day, she felt that you were so concerned about him, and he didn't reply, so she felt a little guilty, so she politely replied to you and asked you to take good care of yourself. This is a refusal to you. The third possibility is that this girl is a confused person, and it's meaningless to just say "take care of yourself" casually.