I find it difficult to read my HRM when I'm asleep.
I try the lay-down-and-rest and the early-waking methods and got it reliably down to 52. It's been lower, perhaps 50 or 51, but I couldn't sustain it for more than a momentary reading. One time I'd fallen asleep with the HRM and woke up in the middle of the night, and I _thought_ it registered lower, but I was so groggy I couldn't remember clearly.
There's a moment of semi-deep sleep where it feels as if my heart takes extra slow, extra powerful beats