Temperature seems to have a big impact on HRV. As I posted before, a very hot bath readily drops my HRV to 1. Previously I speculated a connection between pushing heart rate to max and an HRV of 1, but since have achieved an hrv of 1 at heart rates well below max and have managed a little hrv, as high as 10, when hitting max heart rate.
On the other end, cold dramatically escalates hrv. Here is this morning's reading taken at 62F. And this is after a brutally intense workout the day before yesterday that I'm not fully recovered from.