Translation:Bye, good night.
さようなら also isn't really "good bye" so much as "farewell." I'm by no means a native speaker, but to me this sentence suggests something like a parent tucking their child into bed, knowing that they'll have to leave before the child wakes up and may never see their kid again.
There are less dramatic possibilities than this, but, regardless, さようなら is a pretty dramatic word. So context is an interesting question for this sentence.
Farewell and goodbye are synonyms, aren't they? Since goodbye is a contraction of "god be with you", you're expected to "fare well" with god watching over you. As a native English speaker, this is how I've always understood it. Farewell is a little bit archaic, of course, but I don't think sayonara is considered archaic. What's the distinction between goodbye and farewell for you?
Well, it can be, but "good night" (with the space) is the more common and historical spelling: https://english.stackexchange.com/questions/200743/good-night-vs-goodnight-vs-good-night
Something I learned from Genki is that 『さようなら』is rarely ever used in common speech, as it indicates that you will not see the other person for a long time or until they have turned another chapter in their life. It said that saying 『さようなら』is considered quite dramatic and reserved typically for students saying goodbye to their teachers. Kind of like saying "farewell" in English.
Especially if you phrase it as "have a goodnight", "good night" would need to be two words. When it's a standalone exclamation, "goodnight" as one word is acceptable, but "good night" (with the space) is the more common and historical spelling: https://english.stackexchange.com/questions/200743/good-night-vs-goodnight-vs-good-night
When speaking to someone is it OK to use おやすみ (Oyasumi) without "-nasai"? As far as I understand "-nasai" and "-kudasai" are used to indicate an order or request with "-nasai" being the informal polite and "-kudasai" used as the formal polite (https://crunchynihongo.com/nasai-vs-kudasai/)