Glossary
Parasympathetic nervous system
Also called parasympathetic, rest and digest
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.
The autonomic nervous system Autonomic nervous system 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. More about Autonomic nervous system has two main branches that often pull in opposite directions. The parasympathetic branch handles “rest and digest” jobs: slowing the heart to save energy and getting the gut ready to process food. It takes the lead during quiet, restful moments.
Its main route to the heart is 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 , which carries about three quarters of all parasympathetic fibers. Parasympathetic signals act on the heart in under a second, much faster than 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 ones. Sudden changes in heart rate from one beat to the next are therefore mostly parasympathetic. The heart also speeds up a little when you breathe in and slows when you breathe out, because the vagal brake eases and returns with each breath.
This is what most HRV numbers are built to capture. 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 and 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 respond mainly to parasympathetic activity, so a reading that is higher than your usual tends to mean more parasympathetic influence at that moment. That does not make higher automatically better; the lesson Why higher HRV is not always better explains why.
The lesson The nervous system behind HRV explains how this branch and the sympathetic branch share control of the heart.