Even though I had a good eight hours sleep last night, my wakeup HR was 56. My body is still recovering from my lack of sleep earlier this week. I'm taking another rest day today.
When I ran marathons in the early '80s, my experience was that a night or two of insufficient sleep required about two weeks of good sleep to get my HR back down. Now, it takes less time since I'm not subjecting my body to the stress of long distance running.