Agreement between Fitbit Versa 4 and a reference standard on time-domain nocturnal heart rate variability: A pilot study.

Background: Reduced heart rate variability (HRV) has been associated with impaired autonomic nervous system (ANS) adaptation, suggesting that HRV could serve as a biomarker for identifying ANS imbalances that may adversely affect an individual's overall health. Many wearable devices used outside medical settings provide measures of HRV, but clinimetric data on their reliability and agreement remain limited. The primary aim of this study was to assess the ability of the Fitbit Versa 4, a consumer-based wearable, to measure nocturnal HRV compared to a reference standard HRV measure obtained from an ambulatory ECG device, the Firstbeat Bodyguard 3. Methods: Eight participants simultaneously wore the Fitbit Versa 4 and Firstbeat Bodyguard 3 beginning at least one hour before sleep, throughout their sleep period, and for at least one hour after waking, over a minimum of three nights. HRV was quantified using the root mean square of successive differences (RMSSD). A t-test assessed differences in median HRV between devices, while a Linear Mixed Model and Bland-Altman analysis evaluated relative difference and agreement, respectively. Results: The median RMSSD from Fitbit (21.660 ms [15.927, 22.821]) was not significantly different ( Conclusions: Strong correlation and acceptable agreement between Fitbit Versa 4 HRV measurements and reference standard HRV measurements derived from ECG provide preliminary support for using Fitbit Versa 4 to measure nocturnal HRV outside laboratory settings.