Bradycardia

The sleep study,and recommendations from it, seem to be help for you in the near term.

You might keep notes of your sleep, heart rate, oxygen, headaches, etc., to show the sleep lab.

Best wishes

What is your heart rate during the day? Has your cardiologist considered a pacemaker.

Low HR at night is normal and desirable. It 'should' mean you're in good health and your heart needn't be alerted or aroused often to keep you alive autonomically. However, there is such a things as bradycardia, and it usually results in feeling off, unwell, breathless, maybe fainting or feeling dizzy. Before it gets that bad, you should seek help and get assessed. You may find a pacemaker is in your future. You and literally millions of others around the globe. Millions upon millions.

You might have sleep apnea, but that would normally be accompanied by a rapid heartbeat as your heart is responding to low oxygen levels and trying desperately to keep you alive. So, a low heartbeat would seem to rule out apnea, although the formal diagnosis would still be a good idea. We're all different.

My sleep study showed Very low air flow / oxygen recommended Cpap
Results Great air flow now

My dad and I live in SE Asia. Our local pharmacist provides lower end medical advice and prescriptions. It's our first line of defense. He suggested an oxygen generator, as my dad's episodes of low O2 were noticed during the day and the oxygen was an immediate fix. I have no idea what western doctor would say about this, but for my dad it has worked quickly on the few occasions that he has felt lightheaded and dizzy. His O2 reading is usally around 80-82. It was expensive for us, around $900 US, but dad is happy with it.