I got the below explanation from DC Rainmaker. I think it might clear this up...
The lowest HR of your day is commonly referred to as your resting heart rate (RHR). Now technically speaking, per most…
The RHR will come from the reading while you sleep. If you dream a lot and heart rate is elevated during that time, don’t be surprised to see your HR reading when you wake up lower than the recorded RHR. It doesn’t matter much, the trend is your friend!
I am 45 and in decent shape and I rarely get below 52 on my resting heart rate. Maybe you have better genetics? I've always heard that a lower resting heart rate was a sign of good fitness but I'm definitely not an expert. I also don't fully trust wrist based heart rate monitoring. I find at least for exercising that the chest strap is significantly more accurate. It's not even close.