Since the human heart beats at a non-constant rate, Heart Rate Variability (HRV) is the measurement of this inconsistency, or "the variance in time between the beats of your heart".
This article provides a brief overview on what HRV means and how WHOOP measures it.
For an in-depth analysis about HRV, visit our Ultimate Guide to Heart Rate Variability.
Although HRV manifests as a function of Heart Rate, it originates in the nervous system. Therefore, HRV provides unique information from the information contained within an individual's Resting Heart Rate (RHR).
The autonomic nervous system is the branch of the human nervous system which controls involuntary aspects of our physiology.
The autonomic nervous system consists of two sub-branches:
- sympathetic (activating)
- parasympathetic (deactivating)
Sympathetic activation is a response to stress, exercise, and disease, and causes an increase in Heart Rate. HRV emerges from the interplay between these two competing branches.
Parasympathetic stimulation reflects inputs from internal organs, like the need to digest after eating a meal, and causes a decrease in Heart Rate.
In a balanced nervous system, our hearts are constantly getting mixed messages – commands to increase Heart Rate from the sympathetic nervous system and commands to decreases Heart Rate from the parasympathetic nervous system. These mixed messages cause the resulting Heart Rate to be in a constant state of fluctuation.
When and how does WHOOP measure HRV?
WHOOP calculates your HRV using a "dynamic" average combined with your RHR each night while you sleep - focusing on your last slow-wave sleep stage, or the period when you're in your deepest level of sleep. This calculation method enables you to get an accurate understanding of your baseline, from which you can monitor trends/abnormalities.
For audio learners, check out our WHOOP Podcast No. 29: Heart Rate Variability (HRV).