First of all, you have posted this on the discussion forum, so there isn't any need to apologize for what time you post- it's just an online forum, which people can read at any time they want- it's not like you're waking anyone up.
Secondly, there is no reason for you to feel you have to answer inquiries or requests that come in the middle of the night unless you want to. I turn my phone ringers off when I go to sleep and if I see a message when I get up, I answer it then. You have 24 hours to respond to inquiries and requests, you don't need to respond instantly.
With a Inquiry, all you have to do is message the guest back within 24 hours- you don't need to either pre-approve or decline. If you feel okay about the inquiry and want to pre-approve it, so the guest can go on to book, that's fine, but if you don't want the booking, just message back as to why it's not possible- you don't need to click on decline and since all that's required on an Inquiry is messaging back, don't waste declines on Inquiries, as declines lower your acceptance rate.
If it's a Booking Request, as opposed to a Inquiry, you do have to either accept or decline within 24 hours, but if you have questions to ask the guest, or things to work out with them, you can message back and forth before deciding whether to accept the booking.
So to avoid dings in response rate:
Inquiry- Message back, or Pre-approve within 24 hours
Request- Accept or Decline within 24 hours.
Don't use the terms request and inquiry interchangeably, because they aren't the same thing.
By the way, there is a glitch currently happening where hosts are seeing their response rate steadily go down even though they are answering all inquiries and requests on time- it has been reported and supposedly Airbnb is working on fixing it, so if you see your response rate going down for no reason, don't freak out.
@Gisselle10