My father's ECG is normal. But when I checked his heart rate through Apple watch, sometimes for 5 mins. His heart rate is 33-40 when sleeping. Normal?

It depends. The heart rate tends to get slower as one gets older. This especially occurs at sleep. It also is slow in even younger people who exercise such as running. An EKG should be done to confirm that it is just sinus bradycardia which could be normal. If he is otherwise well, it probably is not a problem. Does he have a doctor. If there were a question some times we have a patient wear a monitor.

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