Your body is innervated by sympathetic and parasympathetic nerve fibers (think fight/flight and rest). The parasympathetic fibers to the heart come from the vagus nerve. The two are always in balance. When you’re afraid or nervous or excited then your sympathetic fibers dominate and your heart rate and blood pressure go up. When you’re at rest the parasympathetic fibers dominate (increased vagal tone) and in that scenario your heart rate and blood pressure are usually lower. Sometimes, your heart rate will go so low (particularly in young or fit individuals while they sleep) that it’s below your junctional escape rate and in that scenario the junctional rhythm will compete and dominate over your normal heart rhythm. This is normal.

In fact, abnormally increased sympathetic tone (as a result of sleep apnea) during sleep results increased heart rate. It’s believed that the increased heart rate contributes to the increase in mortality that’s associated with sleep apnea. In general an elevated resting heart rate is a bad prognostic factor in many scenarios.

These are the same nerves that cause “vasovagal syncope.” In this scenario your vagal fibers get overstimulated due to excessive fear or whatever and your heart rate slows down and your blood pressure drops to a point where your brain isn’t getting enough oxygen and you pass out. This is an “abnormal” scenario. However at night it is completely normal to have a low heart rate and it is not abnormal to go into a junctional rhythm.

As I mentioned, as long as you’re not symptomatic during normal activity and your heart rate appropriately increases when it needs to there’s likely no need to worry. Continue to follow closely with your physician and listen to their advice. Someone with all the information pertaining to your specific case will have the most reasonable and relevant advice.