Large dip in sleeping heart rate

[PeterT7]

I've only had my Fitbit Charge 4 since last month.

Normally, when sleeping, my heartrate bottoms out at 40 or 41bpm, occasionally 39. The trend throughout the sleep is below 50, with gently forays above. Last night I had three interesting spikes, the first was 41-51-58-38-43 in consecutive five minute steps. Then there were five-minute steps of 47-37-37-43, shortly followed by 43-54-62-40. I don't really mind the quick increases, as movement will induce that, but normally my heartrate goes down very slowly (for example, after a long run it will be a few hours before it drops below 60).

It could be timing variations in my Fitbit. Is it normal for a drop of 20 (especially when going down to a record low)?

I've had an leg injury and so avoided even walking for a while now and I am planning on a file-miler tomorrow, as long as the calf muscle doesn't tense (it did in last Wednesday's walk). The lack of recent exercise means that I am surprised at the record low.

Moderator Edit: Clarified subject

[YojanaFitbit]

Hey there, @PeterT7! Welcome to the Family!

Thanks for sharing all those details in your post. As to your question about resting heart rate, it's difficult to say. I think it's best to check with a doctor to make sure it's okay. Are you having same readings lately? 

Looking forward to see you again in the near future.