When I ran seriously (40-60 miles/week, late teens/early 20s) my heart rate at rest was around 43, so it's fair to assume it dropped lower whilst asleep.
Doing more aerobic exercise should improve your general cardiovascular efficiency and possibly drop your blood pressure, a fall in heart rate at rest is to be expected.
Provided you feel fit and well you're probably worrying unduly. Perhaps the Apples watch is doing you more harm that good? Hell will freeze over before I`ll entertain owning one of those bloody things. Do yourself a favour, put it in a drawer, leave it there. These things turn folk into hypochondriacs.
If you seriously have concerns you shouldn't be increasing the running until you've had a proper health check.