automatic nap detection

Works fine for me. Efficient nap defined by garmin as less as 30 min sleep. Not sure but maybe less than 60 min sleep automatically detected as a nap...

What are the requirements and settings for successful nap detection?

No settings. Go to sleep. 

Does the HR measurement not have to be active?

I think it should.
NAP detection works quite good for me.
BTW, detecting not only NAPs but also REST periods.

Watch should register naps automatically. In my case, it registered sitting in relaxed state as a 13 minute nap.

I haven't understood it yet.
Sitting quietly should of course not be considered a nap, but to determine that you are sitting quietly, the HR measurement must be activated?
So the HR measurement must be switched on for detection and I have only switched it on at night. No wonder the detection doesn't work for me.
But Garmin could have made this clear.

I suppose HR needs to be activated, but it is just a guess. I have it enabled and nap detection works (although with above mentioned glitch).

It should be easy to test - just start nap manually from the widget and see whether HR enables then. If it does, it means that HR is mandatory for nap to register.

ok so it seems it should work automatically, thank you for confirmation.

Somehow with my first test it's not triggered at all. 

Now it's matter of investigation why in my case nap is not detected.