Alarm every midnight on iPhone 13 and iPad air.

For a while now every night at midnight exactly my iPhone 13 sounds a tritone alarm.

I have no alarms set. I have turned off all notifications. I have updated my phone (to iOS 16.1.1). I have factory reset my iPhone and restored from back-up. I have factory reset my iPhone and manually reloaded apps from the App Store. I have deleted the Health app (so no bedtime settings.) I have deleted the clock app (alarm still sounded, so I downloaded it again.) There are no reminders set, including in the Calendar app. I have put the phone on silent, alarm still sounded.) I have never used the tritone setting for an alarm or notification I have set (I have my own custom tones I always prefer to use. Siri is completely shut down on every app.

Last night I left my phone downstairs but my iPad Air 2 which I had by the bed sounded the same alarm at midnight. Have again checked all settings on the iPad, same as iPhone, but I haven't updated the software, it's still on iOS 15.1.

There are no banners, flags, notifications or other indications as to what is causing it.

I am at my wits end. I've probably done other stuff not listed above, just can't remember it all!

I have searched and found other references to a similar problem but I think I have tried every suggestion that has been made in those cases. But I'm open to any suggestions people can make, apologies if I have to say "tried that too!"

I'm starting to really suffer from my sleep being broken every night.