I Am A 62 Year Old Female In Relatively Good

I am a 62 year old female in relatively good shape for my age. I recently started wearing a
Fitbit device that measures my heart rate. it says my resting heart rate averages around 64 BPM. But, seems it drops to 51-56 when I am sleeping. is that normal? I have concerns as there is heart disease in my immediate family. I did make an appt with my doctor, but feel silly if it is normal for your heart rate to drop this low during the night. your opinion is appreciated. I hate going to a doctor for a silly reason!! my e-mail address is: