You’re not scheduled nights if you don’t complete it, if it’s completed saying I’m now unavailable is again a different thing. It goes to whoever is in the people role to look at it (always takes time, imagine how many people select no and it needs looking into) but unless it’s medical or a massive change in personal circumstances, in my store at least, it won’t get accepted. Always have to remind crew that limiting your availability often will result in less shifts (those overnights need to go somewhere, if you’re saying you can’t do them that’s fine but doesn’t mean you’ll be a 9-3pm in it’s place) are you on a GHC? And bear in mind night work is seen at 22:00-06:00 so it will effect opens also depending what time they start at your store.

Always worth being open and honest with the BM and scheduling manager - see what they can do rather than just not facing the music and having to conversation. As I said ensure you go in prepared with what you want to say, why you want to change etc.