How is HRV calculated?

Heart Rate Variability (HRV) is calculated by measuring the time intervals between successive heartbeats. Unlike a metronome, the heart does not beat at perfectly regular intervals. The variation in these intervals is what constitutes HRV.

HRV can be measured in two main ways:

1. Time Domain Analysis: This involves looking at the standard deviations and variations in the time between each heartbeat. Different indices like the Root Mean Square of Successive Differences (RMSSD) are commonly used for this measurement 1.

2. Frequency Domain Analysis: This method looks at the frequency of changes in heart rate, categorizing them into high and low frequencies to infer parasympathetic and sympathetic activity respectively. High-frequency variations indicate more parasympathetic (relaxation) activity, while low-frequency variations correspond to sympathetic (stress or activation) activity 2.

Most modern wearable devices, like the WHOOP and Oura Ring, measure HRV using similar principles but vary in specifics depending on the technology used. For example, WHOOP typically takes HRV readings during the last episode of slow-wave sleep to provide a meaningful measure of recovery and readiness for the next day 3.