1. the appropriate departure phrase for leave-taking after dark. (This assumes that the speakers will not see one another until morning at the earliest. Night alone is familiar.) John: Bye, Alice. Alice: Night. See you tomorrow.Bill: Good night, Mary. Mary: Night, Bill.
2. the appropriate phrase for wishing someone a good night's sleep. Father: Good night, Bill. Bill: Night, Pop.Father: Good night. Mother: Good night.