Why he doesn't reply?

A guy has been chasing after me for some time. Im quite sure about that bcoz of his attitude and he has asked me out for several times, we always text each other and sometimes chat on the phone. I know my attitude has been somehow ambiguous towards him coz im not quite sure if this relationship is wt i want and i want to observe more. But honestly i care about him. Yesterday we were texting each other as always, talking about exams and stuff. I msged him saying "dont stay up too late" and after that he hadn't reply me. I feel pretty worried coz this is the first time he doesn't reply to my text. Why does he do that? Is that wrong for me to tell him that? :(
Why he doesn't reply?
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