Dr. Boris,
Thank you for your response. I did take him to his pediatrician who recommended he see a Pediatric Cardiologist to be on the safe side. The Cardiologist did an EKG and an Echo. He diagnosed sinus arrhythmia and said that everything else looked normal.
Dear Resting,
The most common reason for an irregular heart rate in a child is what is a normal finding called respiratory variation (or sinus arrhythmia, if reading it on an ECG). The heart rate increases with inspiration and decreases with expiration. Without evaluating your son, I cannot say if he has this or something more concerning. I can say that his nighttime heart rate readings suggest that he does have increased vagus nerve tone, which can slow down the heart rate. This can be more associated with respiratory variation, as well. As long as he has adequate blood flow to his brain and kidneys, I am not so concerned with the actual heart rate being in the 50's-60's. This is not likely to be long QT syndrome. Heart block is probably also of low likelihood here. I would check in with the pediatrician first and see if the ECG was read as sinus arrhythmia. If there are early beats (premature atrial or ventricular contractions), then it may be more appropriate to be seen by a pediatric cardiologist for this.