SDARR is the standard deviation of the set of averaged RR intervals; this averaging period is typically 5 minutes. If the entire sample period is 5 minutes, you will have a set containing precisely one averaged RR interval. The standard deviation of a set containing one element is always 0.
Note: The default averaging period is 5 minutes. This can be altered in the HRV Settings dialogue “SDARR” averaging text box (see image below).
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