Depending on what I eat usually determines how many times I have to go in the middle of the night. On average it is around 3 BM a night. As others have said, sudden movements and sleeping on my left side bring on the "Uh oh, gotta go" feeling. I don't sleep completely flat on my back, if I do lay down on my right side, I am ok. Flare or nonflare, this has been status quo for me since I was diagnosed. It doesn't interrupt my sleep too much. I usually go, then fall back to sleep right away.
Honestly, I rather have it this way than constantly having to go all the time during the day.