GoKawiilGoogle explains how Fitbit calculates heart rate variability
Google detailed how Fitbit devices measure heart rate variability, using an optical sensor to estimate gaps between heartbeats and applying the RMSSD formula to produce a millisecond-based score. The metric draws on the longest sleep period of at least three hours within the prior 24 hours, and results appear in Google Health under Health > Vitals > Heart rate variability, showing daily and long-term trends plus a personal range from up to 30 days of data.