안녕히 주무십시오(answered by Sakura) is a little too formal, actually I rarely say it. You can say 안녕히 주무세요 when you say it to your parents, teacher and so on. 잘 자(Sleep well), 좋은 꿈 꿔(Have a good dream) are what I say to my friends. You can say 잘 자요, 좋은 꿈 꾸세요. if you have to use the honorific. Hope this helps.

annyonghi chumusipsio...that is base on my research...i dont know how to speak korean but i want to help you...