iPhone beeps for a short time at the same time every day

Despite being on silent, plus Bed Time focus, and having no alarms on, my iPhone (14, iOS version 18.5) makes a beeping sound every night at 11:57pm. If I raise my phone to see the screen there is no alarm or notification visible, just the bed time focus screen. The beeping stops after about 30-45 seconds. I have turned off all notifications from having sound enabled except messages and phone calls. I have no idea how long this has been going on as I only became aware of it recently when my boyfriend pointed it out (I'm usually fast asleep).

Any insight into what this might be and how to stop it?