HRM not recorded at night

What is your Bangle.js firmware version, and are the apps all up to date if you go to https://banglejs.com/apps/?

And are you sure you've removed the protective plastic off the bottom of the HRM sensor?

When I had HRM during the night, I didn't have it in Gadgetbridge on my phone

What do you mean here? That you saw you had HRM recorded over night in the Health app's graph, and yet it didn't appear on your phone in Gadgetbridge?

If anything the heart rate monitor tends to guess the heart rate, so about the only way it'd stop recording heart rate is if the proximity sensor on the back didn't think the watch was being worn at all.

Please could you connect with the Web IDE and then paste this code into the left-hand side:

Bangle.setHRMPower(1);Bangle.on("HRM-raw",h=>g.clear(1).setFont("6x8").drawString(JSON.stringify(h,null,2)));

Specifically I'm interested in the vcPS value that's displayed on the screen when you're wearing normally and when you're pressing the watch onto and on the side of your arm as if you might be lying down.

Normally for me, vcPS is at 15 pretty much all the time when I'm wearing the watch, but if for some reason yours is dropping to below 7 the watch could be thinking it's not being worn and so not recording a value