HRV is controlled mostly by your nervous system. 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 acts as a fast brake on the heart, 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 nerves act as a slower accelerator. At rest, HRV mainly shows how active the brake is.
In lesson 2 you saw that HRV is the wiggle in a chart of the gaps between beats. This lesson explains what makes the line wiggle, and what that does and does not tell you.
Your body’s autopilot
You don’t decide how fast your heart beats, how fast you digest food or how wide your pupils open. A part of your nervous system handles all of this without you thinking about it. It is called 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 , which means “automatic”.
It has two main branches:
- The sympathetic branch gets you ready for action. It is often called “fight or flight”. It speeds the heart up and makes each beat stronger.
- 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 looks after rest and recovery. It is often called “rest and digest”. It slows the heart down, which saves energy when you are resting.
Both branches are active all the time. Think of them as two hands on the controls, each pressing a little harder or a little softer from moment to moment.
The main nerve of the parasympathetic branch is the vagus nerve. It runs from the brainstem down through the neck and chest to many organs, including the heart. About three quarters of all parasympathetic fibres travel in it.
The brake and the accelerator
Your heart has its own pacemaker, the sinoatrial node. Left completely alone, it would fire about 100 times a minute. Yet most people at rest have a heart rate well below that. The reason is the vagus nerve, which keeps a steady foot on the brake.
The sympathetic nerves work the other way. They press the accelerator, raising heart rate during exercise, excitement or stress.
The two also differ in speed:
| Vagus nerve (parasympathetic) | Sympathetic nerves | |
|---|---|---|
| Effect on heart rate | Slows it down | Speeds it up |
| How fast it acts | In under a second | More than 5 seconds to start, 20 to 30 seconds to reach full effect |
| When it leads | At rest, during sleep, when calm | During exercise, stress, excitement |
The vagus can change the timing of the very next heartbeat. The sympathetic nerves are too slow to do that. So the quick jumps from one gap to the next mostly come from the vagus.
HRV at rest mainly reflects vagal (parasympathetic) activity, because the vagus nerve can change the timing of the very next heartbeat. This is why researchers use 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 , the measure you met in lesson 2, as a sign of vagal activity Vagal tone 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. More about Vagal tone .
Go deeper How the nerves talk to the heart
The vagus nerve releases a chemical messenger called acetylcholine at the pacemaker. It slows the rate at which the pacemaker fires. The sympathetic nerves release noradrenaline (also called norepinephrine), which speeds the pacemaker up and makes the heart squeeze harder.
Breathing in, breathing out
In lesson 1 you felt your pulse speed up as you breathed in and slow down as you breathed out. Now you can see why. When you breathe in, the vagal brake eases off a little, and the heart speeds up. When you breathe out, the brake presses again, and the heart slows. This pattern is 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 .
Because of this, how you breathe changes your HRV. Slow, steady breathing makes the wave in the tachogram Tachogram A line chart of heartbeat intervals in the order they happened, one point per beat. It shows HRV as a wavy line instead of a single number. More about Tachogram bigger. For most adults the biggest swings come at a pace between about 4.5 and 7.5 breaths a minute, often close to 6. That is a slow count of 5 in and 5 out. At this pace, breathing, heart rate and blood pressure can fall into step with each other, and the rise and fall in heart rate grows.
Try it with the pacer below. Press Start and follow the circle for 2 minutes. Stop and breathe normally if you feel dizzy or light-headed.
A larger swing during slow breathing is a normal response. It does not mean slow breathing treats any health condition. If you want to know what the research says about breathing practice over weeks, read can breathing exercises raise your HRV.
What “high HRV” means for your nervous system
At rest, a higher HRV than usual for you most often means the vagal brake is more active. That fits with rest, recovery and calm.
Be careful with the word “stress”, though. HRV tracks your nervous system, and plenty of things besides worry change it: your posture, your breathing, talking, moving, a meal or the time of day. A hard workout the day before, a late dinner or a short night can all shift it too. To read more about how worry and mood relate to HRV, see HRV and stress and does anxiety lower HRV.
Why this matters for your readings
Because the vagus reacts within a beat, almost anything you do can change your HRV in the moment. Researchers handle this by measuring under the same conditions every time: same posture, after a few minutes of settling, without talking or moving, breathing normally.
You can borrow that habit. A reading taken while you sit and chat will not match one taken lying still after waking. Lesson 9 covers how to take a good reading.
For the background, see what HRV is. Next, in why higher HRV is not always better, you will see when a high number is not good news.