Low heart rate at the end of the night

[Ella36]

Hi, 

I am a researcher analysing Fitbit Charge 5 heart rate data during sleep. I have noticed a very consistent increase in heart rate during the end of the sleep period across hundreds of nights of data, which coincides with low confidence values (0, 1, or 2). Does anyone have an explanation for why this might happen? Or any information about how heart rate data is generated?

For reference, here is an example of my own testing data (note that the bottom red line shows the onset of sleep and the top red line, the offset) with Fitbit sleep stages on the left and HR (bpm) on the right (colour-coded by Fitbit "confidence" values):

Thank you for any insights!

Moderator Edit: Clarified subject

[Gondwana]

Ah; that's not using SDK or Web API. I'll move topic to Sleep section.

[karlang]

I wonder if movement or changes in position during lighter sleep stages could be affecting both the heart rate readings and the confidence levels. It might also have something to do with how tightly the device is worn or skin contact overnight.