Stress tends to lower your HRV while it lasts, because the vagal brake on your heart eases off. Slow breathing at about six breaths a minute does the opposite in the moment: your heart rate swings up and down with each breath, and HRV grows. Neither turns HRV into a reading of how you feel.
Lessons 11 and 12 covered alcohol, illness and training. This lesson is about the load you carry in your head, and the one tool that changes HRV within a minute: your breath.
How stress shows up in HRV
Remember the brake and the accelerator from lesson 3. When you face a deadline, an argument or a scare, the vagal brake eases off. Your heart speeds up a little and its rhythm becomes more even. HRV goes down. When the moment passes, the brake comes back and HRV recovers.
A 2018 review of 37 studies found that HRV changed in response to stress in most of them (Kim and colleagues, 2018). The most common finding was lower 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 activity.
Rest, reactivity and recovery
Researchers often look at HRV in three steps, sometimes called the three Rs (Laborde and colleagues, 2017):
| Step | What it is | Example |
|---|---|---|
| Resting | HRV at rest, before anything happens | Sitting quietly for 5 minutes |
| Reactivity | The change from rest to a challenge | HRV during a stressful task |
| Recovery | The change after the challenge ends | HRV in the minutes afterwards |
Your daily reading is a resting measure. It cannot see your reactivity during a hard meeting, only what is left over by the time you measure.
Everyday stress is harder to see
A Finnish study measured 99 employees at home (Hynynen and colleagues, 2011). People who reported more stress symptoms had lower HRV in a short morning test. Their overnight HRV, though, was no different from the low-stress group’s. So depending on how and when you measure, stress you feel may or may not show up.
Device “stress scores” are built on this signal. Each maker uses its own formula, so scores from different devices do not match. Treat them as estimates.
What slow breathing does
You already know from lesson 1 that your heart speeds up a little as you breathe in and slows as you breathe out. At normal breathing rates, those swings are small.
At about six breaths a minute, your heart rate swings and a blood pressure reflex start to move in step, and the swings grow to many times their resting size. Lehrer and Gevirtz (2014), who reviewed this research, say it happens in almost everyone, often within a minute, even on the first try. The best pace differs a little between people. In their work it averaged about 5.5 breaths a minute.
Try it with the pacer: about 4 seconds in and 6 seconds out. Breathe gently, not deeply. Stop and breathe normally if you feel dizzy or light-headed.
What it does and doesn’t mean
A huge HRV number during slow breathing mostly shows the exercise working. Lehrer and Gevirtz note that slower breathing and longer out-breaths can make the swings bigger, possibly without any change in 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 . So the number during the exercise is not your resting HRV.
What about afterwards? A 2022 meta-analysis pooled 223 studies of slow breathing (Laborde and colleagues, 2022). Vagal HRV was higher during the breathing, right after single sessions, and after programmes of practice lasting weeks. The studies of longer-term effects were fewer. The authors described slow breathing as a low-cost technique with few side effects expected.
Breathing with feedback
HRV biofeedback HRV biofeedback A breathing practice in which you watch your heart rate live and breathe slowly, often near 6 breaths a minute, to make the heart's rise and fall with each breath as large as possible. More about HRV biofeedback means breathing slowly while watching your heart rate swings on a screen, so you can find the pace that makes them largest. A 2017 meta-analysis of 24 studies with 484 people found it was linked with a large drop in self-reported stress and anxiety (Goessl and colleagues, 2017). The authors also said more well-controlled studies are needed.
Go deeper Why your breathing changes some HRV measures more than others
The fast up-and-down with each breath is called respiratory sinus arrhythmia. Frequency measures such as HF power depend on breathing rate: they only capture the vagal part properly when you breathe roughly 9 to 24 times a minute. RMSSD is less affected by breathing rate, which is one reason Laborde and colleagues (2017) recommend it.
For your daily reading, the advice is simple: breathe normally. Do not try to slow down or speed up, or you will change the number you are trying to measure.
Using breathing without spoiling your data
- Keep your reading routine fixed. Take your daily reading at the usual time, in the usual way, and never during or right after a breathing session.
- Practise at another time. A few minutes before bed or during a busy afternoon works.
- Judge it over weeks. After a few weeks of practice, compare your 7-day average with the weeks before. Expect a small change at most, because age and genes set much of your range. See can breathing exercises raise HRV?
- Notice how you feel. Feeling calmer is a good result on its own, whatever the number does.
When to talk to someone
Slow breathing is safe for most people. If you have a heart or lung condition, or breathing exercises make you dizzy or anxious, ask your doctor first. Breathing practice can support treatment for anxiety but should not replace it. If stress, worry or low mood affects your sleep, work or relationships for more than a couple of weeks, talk to a doctor or a mental health professional, whatever your HRV shows. For more, see does anxiety lower HRV?
Next, in limits, myths and red flags, you will learn what HRV cannot do, and when symptoms matter more than any number.