[Evans, Marv]

You are correct in saying they only go off in the middle of the night. I can't understand why either. Mine used to do that & scare the shyte out of us. They're hard wired, and when I turned off the circuit at the main box and just let them run on battery, we didn't have any more problems. They work right now because a few weeks ago my wife created some smoke while cooking, and they all went off. I tried changing out one a couple of years ago I thought might be defective, but it made no difference. I'm thinking when I change them all out for new ones, it's just going to be the battery powered type.