Fitbit HRV readings reveal personal stress and recovery trends

Fitbit’s heart rate variability (HRV) reading is best understood by comparing it with your own usual level, rather than another person’s. HRV measures small differences in the time between heartbeats and can offer clues about how your body is responding to stress and recovery, but a lower reading alone does not show that you are ill.

Fitbit estimates HRV with its optical sensor and calculates it using RMSSD, in milliseconds, from the longest qualifying sleep period in the previous 24 hours. Readings can shift with sleep, strenuous exercise, stress or alcohol; Fitbit also tracks longer-term trends and a personal range based on up to 30 days of data. A 2024 study found moderate to strong agreement between the Fitbit Charge 4 and reference measurements, though Fitbit tended to underestimate HRV; persistent changes, especially alongside symptoms, are a reason to speak to a health care provider.