Glossary
Sympathetic nervous system
Also called sympathetic, fight or flight, fight-or-flight
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.
The sympathetic branch of 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 gets the body ready for action. During exercise, fear or excitement it raises the heart rate and the force of each contraction, so the heart pumps more blood each minute. It also widens the airways and moves blood toward the muscles.
On the heart it works slowly compared with 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. After sympathetic stimulation starts there is a delay of up to 5 seconds before heart rate begins to climb, and it takes 20 to 30 seconds to level off. Parasympathetic signals, by contrast, act within a beat. Because of that timing, the quick, beat-to-beat changes that 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 measures come mostly from the parasympathetic side.
So no common HRV number gives a clean readout of sympathetic activity. The LF/HF ratio LF/HF ratio LF power divided by HF power. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio was once reported as a gauge of the balance between the two branches, and many researchers have since challenged that use. The two branches also do not always move like a seesaw, with one rising as the other falls.
The lesson The nervous system behind HRV covers both branches. For the everyday things that move HRV, see What affects HRV?