Does the heart ever rest?
Yes and no.
Cardiac muscle is myogenic,meaning it contracts involuntary independent of any input from the nervous system.
You could say the heart rests due to atrial and ventricular diastole during the cardiac cycle,when the atria and ventricles do "relax"(not contract),to allow the chambers to refill with blood .
But this is not really resting as it is over in mere seconds,and when either the atria or ventricles is in diastole,the other one is contracting,so the whole heart is never truly resting.
If the heart rested(stopped contracting,that would lead to cardiac arrest .