I'm having similar but slightly different experiences. Maybe someone can shed some light, or suggest something and we can both become better infomred.
My resting heartbeat gets down into the low 50's sometimes. It seems to be regular and I suffer no ill effects like dizzyness, passing out, etc. I do get tired, but I've been on a diet for about 4 months now to lose weight (successfully so far, but still 20% overweight), so I've been attributing late night fatigue to lack of calories. I briskly walk 5 to 6 miles per day, 6 days per week. My heartbeat goes to 70% to 85% of maximum, and my recovery rate is within 10 minutes. I'm tired when finished, but not where I'm passing out or wanting a nap. I usually go back to work after the workout and function just fine.
A few questions: Can dieting slow your hearbeat down? I know exercise makes your heart stronger, but into the low 50's?
Any help or insight with this would be greatly appreciated.