Hello, thanks for the question. We commend you for tracking your heart health. Sometimes apple watches aren't as accurate as we like but they do give an idea. You are probably aware that night time and in particular 10 to 2 is the period of greatest variability. A general rule is that your night time HRV should be 20 to 30 percent lower than your day time resting heart rate. 40 to 50 beats per minute is a common HRV at night. Everyone is different though so an absolute number is hard to give.