I need advise on my retainers; please read my situation.?

Hello everyone. So I got my braces taken off. It's been months (I'm not sure how long exactly). Anyway, my orthodontist put me permanent retainers on my bottom but he refused to put permanent on my upper teeth as well because he said that my bottom teeth would break the retainers on top when i bite. So he gave me clear retainers for my top teeth and told me to wear them ONLY at NIGHT for not less than a year. The question is: I don't know if I should wear them night AND DAY, like not just at night, because I am afraid my teeth will shift. So what would you recommend? Thank you so much for your help, I really appreciate it! XD