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A client sat down across from me a while back, held up her wrist, and said, "My watch says my HRV was low all week. Is my nervous system broken?"
I wrote a book with "polyvagal" in the title, so you would think I'd have a tidy answer ready. I do have an answer. It just isn't tidy, because the honest one never is: heart rate variability is a real, useful physiological measure that our field doesn't understand, use, or teach its professionals about.
Clients bring us wearable data now. Consultation groups toss around "vagal tone" like everyone agreed on a definition. So let's actually get clear on what these terms mean, what the research supports, and what I do with all of it in a therapy office.
Vagal tone is the ongoing influence of the vagus nerve on the heart. Your heart's pacemaker would fire around 100 beats per minute on its own; the vagus continuously slows it, like a brake resting on a pedal. Stronger, more flexible vagal influence generally means a system that can shift states efficiently rather than getting stuck.
We cannot measure that influence directly in a clinical setting, so researchers estimate it. The most common window is respiratory sinus arrhythmia: your heart naturally speeds slightly when you inhale and slows when you exhale, and the size of that swing reflects vagal activity at the heart (Shaffer & Ginsberg, 2017). Which brings us to HRV.
Heart rate variability is the beat-to-beat variation in the time between heartbeats. Not your heart rate. The wobble in your heart rate. A heart that beats with slight, constant variation is being actively managed by a responsive nervous system. A metronome-steady heart, counterintuitively, is the worrying one.
Here is where I need you to slow down with me, because this is the part the wellness industry skips. "HRV" is not one number. There are time-domain metrics, frequency-domain metrics, and short-term versus 24-hour recordings, and they do not all reflect the same physiology (Shaffer & Ginsberg, 2017). Recording length, breathing, posture, and context all change the numbers, which is exactly why researchers have published detailed measurement and reporting guidelines: sloppy HRV methodology produces sloppy conclusions (Quintana et al., 2016; Laborde et al., 2017).
A consumer wristband estimating HRV from an optical sensor while you sleep is a very rough cousin of a laboratory ECG. Useful for trends in one person over time. Not a clinical instrument.
| What HRV can tell you | What HRV cannot tell you |
|---|---|
| Trends in one person over time, under consistent conditions | Whether an individual is regulated or dysregulated from a single reading |
| A rough index of vagal influence on the heart, when measured properly | Which autonomic "state" a client is in during your session |
| Group-level patterns across clinical populations in research settings | A diagnosis, a verdict, or proof of any theory by itself |
With those caveats, the body of research here is one reason I remain a nervous system oriented therapist.
Higher resting HRV keeps showing up alongside better emotion regulation. The neurovisceral integration framework proposes why: the same prefrontal circuits that regulate emotion also participate in regulating the heart, so HRV works as a rough index of how much regulatory capacity is online (Smith et al., 2017). A meta-analytic review backs this up, linking higher HRV to stronger top-down self-regulation (Holzman & Bridgett, 2017), and related work has mapped how heart-brain traffic runs in both directions (Mather & Thayer, 2018).
And the clinical populations we sit with show a consistent pattern. Meta-analyses have found lower HRV, on average, in anxiety disorders (Cheng et al., 2022), in major depression (Koch et al., 2019), in PTSD (Schneider & Schwerdtfeger, 2020), and under chronic stress generally (Kim et al., 2018). Some researchers have gone as far as proposing reduced HRV as a transdiagnostic marker that cuts across our diagnostic categories (Beauchaine & Thayer, 2015).
Translation for the therapy room: the bodies of trauma survivors are telling a measurable story. The hypervigilance you see in session has a physiological signature. That validates what our clients have been saying all along, which is that this is not in their heads.
No. A single HRV reading tells you almost nothing about an individual. The research findings are group averages with wide, overlapping distributions, and a person's numbers swing with sleep, alcohol, illness, medication, fitness, age, and menstrual cycle. Low HRV on a Tuesday is a data point, never a diagnosis.
This is where I pump the brakes hardest, and yes, the pun is intended. HRV is not a polyvagal-o-meter. It does not tell you which "state" someone is in, it cannot confirm or refute anyone's theory by itself, and even its relationship to vagal function has documented complications that physiologists still argue about (Grossman, 2023). I have written elsewhere about holding polyvagal theory as significant without treating it as sacred, and the same discipline applies here: my response to the Grossman critique covers that ground.
If a client brings you a scary number from a watch, your job is the same as it always is: get curious about the person, and let the number be one small voice in a much larger conversation.
My rule of thumb: wearable HRV is a mood journal your wrist keeps. Interesting for patterns, worthless as a verdict, and never a substitute for asking your client how they slept, what they consumed, and what is going on in their life.
This is the question underneath every "10 vagus nerve hacks" listicle, and the answer is a qualified yes. Slow-paced breathing, particularly around six breaths per minute with an extended exhale, reliably increases HRV in the moment, and heart rate variability biofeedback, which trains exactly that, has a real evidence base for stress and anxiety outcomes (Goessl et al., 2017; Lehrer et al., 2020; Zaccaro et al., 2018). Regular aerobic exercise and decent sleep are associated with healthier autonomic profiles over time (Laborde et al., 2018). Nothing exotic. The boring fundamentals, doing what the boring fundamentals do.
What I want you to hold onto is the direction of the claim. Practices that engage the vagal brake can build a more flexible system over time. That is different from promising that a higher number will heal trauma. Reprocessing work still has to happen. In my model, autonomic resiliency and memory reprocessing feed each other; neither replaces the other.
Three things, and none of them involve a gadget.
First, I use the concept in psychoeducation. "Your nervous system has a brake, and we can strengthen it" is accurate, hopeful, and shame-free. It turns a client's symptoms from a character flaw into a trainable system.
Second, I track state instead of chasing numbers. Breath location, voice, pacing, eye contact, the quality of contact between us. Your observation skills, refined over thousands of clinical hours, are a better real-time instrument than any wristband on the market.
Third, I build vagal-brake work directly into EMDR preparation. In my Preparation Hierarchy, a client needs to be safe enough to feel before we ask their system to notice, flex, and eventually trust the process of reprocessing. Extended exhale practice, orienting to cues of safety, interoceptive noticing: this is Phase 2 work with a physiological rationale behind it, and it changes who can tolerate reprocessing and how it goes.
The science of the heart-brain connection is genuinely beautiful. It deserves better than hype, and so do your clients.
Want the full framework, taught from the beginning?
On November 19th, I'm teaching Introduction to Polyvagal Informed EMDR, my intro-level course on the model: three live hours on the autonomic nervous system, the honest state of the science, the Preparation Hierarchy, and interventions you can use in your next EMDR session. Three CEs, on-demand access included. Not EMDR trained? Start with Polyvagal Theory Interventions for Trauma Therapists.
I'm Ready to Work With the Nervous System, Not Around It
You belong here.
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Cheng, Y. C., Su, M. I., Liu, C. W., Huang, Y. C., & Huang, W. L. (2022). Heart rate variability in patients with anxiety disorders: A systematic review and meta-analysis. Psychiatry and Clinical Neurosciences, 76(7), 292-302. https://doi.org/10.1111/pcn.13356
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Grossman, P. (2023). Fundamental challenges and likely refutations of the five basic premises of the polyvagal theory. Biological Psychology, 180, 108589. https://doi.org/10.1016/j.biopsycho.2023.108589
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Kase, R. (2023). Polyvagal-informed EMDR: A neuro-informed approach to healing. W. W. Norton.
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Zaccaro, A., Piarulli, A., Laurino, M., Garbella, E., Menicucci, D., Neri, B., & Gemignani, A. (2018). How breath-control can change your life: A systematic review on psycho-physiological correlates of slow breathing. Frontiers in Human Neuroscience, 12, 353. https://doi.org/10.3389/fnhum.2018.00353