What to do about heart pounding? Not rapid: 52bpm. Wakes me up at 4:30 am. Not feeling anxious. Persists more lightly thru day.

Probably nothing. Unless you have high blood pressure, then it should be treated. Another reason could be sleeping on your left side.
?? Serious or not. This may or may not be significant. Laboratory tests to check for anemia and thyroid conditions may be helpful. Also, wearing an event monitor may help determine what is happening when you are having your symptoms.
Blood pressure. You may be more aware of your heart beat if your blood pressure is high. Alternatively, you could have a heart rhythm problem like bigeminy which may feel like a slower pulse with accented beats. Check with your doctor.

Related Questions

What can I do about heart pounding? Not rapid: resting is 52bpm, but it wakes me up at 4:30am. Not feeling anxious. Persists lighter thru day.

Blood pressure. You may be more aware of your heart beat if your blood pressure is high. Alternatively, you could have a heart rhythm problem like bigeminy which may feel like a slower pulse with accented beats. Check with your doctor.