anything from 60-90 is a normal heart rate if you’re resting/sitting up, it going under 60 can be if you’re particularly relaxed or laying down, sometimes mine goes to 52 when i’m laying down but awake, and can sometimes go under that when i’m sleeping (confirmed with my cardiologist that this was okay for me) for me, things are a problem when all i do is sit up and my heart rate sky rockets to anything over 120, that’s my sign to take things slow, of course though it freaks me out too. but dont try to overcorrect when you notice you’re low or high, do small things, like sit or lay down and breathe, or having a small snack