Glossary
Vagal tone
Also called cardiac vagal tone, vagal activity, vagally mediated HRV
The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power.
The vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve works like a brake on the heart, slowing it beat by beat. Cardiac vagal tone is the name for that braking: the contribution of the parasympathetic nervous system Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system to how the heart is controlled.
Outside a lab, vagal tone is estimated from the heartbeat itself, using measures that respond mostly to vagal activity: RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , HF power HF power The share of heart rate variability in fast rhythms of 0.15 to 0.40 Hz, the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power and the breathing-linked rhythm called respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia (RSA). Researchers call these “vagally mediated HRV”. When an app says your HRV reflects your “rest and digest” side, this is the link it is relying on.
The link is useful but loose. Breathing rate and depth change RSA and HF power without any change in vagal tone. Physical activity at the time of measurement does the same. Grossman and Taylor also point out that the link is stronger when you compare one person with themselves over time than when you compare different people. That is a good reason to watch your own trend rather than rank yourself against others.
The lesson The nervous system behind HRV explains how the vagus nerve and the sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system system share control of the heart.