Heart rate variability is literally the variance in time between the beats of your heart. So, if your heart rate is 60 beats per minute, it’s not actually beating once every second. Within that minute there may be 0.9 seconds between two beats, for example, and 1.15 seconds between two others. The greater this variability is, the more “ready” your body is to execute at a high level.
These periods of time between successive heartbeats are known as RR intervals (named for the heartbeat’s R-phase, the spikes you see on an EKG), measured in milliseconds. WHOOP calculates HRV using RMSSD, the root mean square of successive differences between heartbeats.
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