Glossary
Autonomic nervous system
Also called ANS
The part of the nervous system that runs the body without conscious control, including heart rate. HRV mostly reflects how it adjusts the heart from moment to moment.
You do not decide how fast your heart beats or how quickly you digest lunch. The autonomic nervous system (ANS) handles these jobs in the background. It is sometimes called the involuntary nervous system for that reason.
It has two main branches. 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 branch prepares the body for action (“fight or flight”) and speeds the heart up. The parasympathetic 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 branch handles “rest and digest” and slows the heart down, mostly through 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 . The heart receives input from both at once, and they usually have opposite effects. At rest the parasympathetic side leads.
Heart rate variability exists because these two inputs keep shifting. Rhythms from breathing and from blood pressure control nudge them up and down, and the heart’s timing follows. That is why HRV can be used as an indirect view of the ANS.
That view has limits. The Task Force standards note that HRV measures fluctuations in autonomic input to the heart, not the overall level of that input. A low HRV can come from a weaker parasympathetic brake, from very strong sympathetic drive, or from other factors, and the number alone cannot tell you which. Reading HRV as a trend, alongside resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate and how you feel, works better than reading one value.
The lesson The nervous system behind HRV explains both branches in more detail.