Or…better yet, you read the number derived from your wearable device and do absolutely nothing about it.

I have found two camps exist in the health and wellness sphere. First are those who are biometrically hypervigilant. They wake up each morning and instead of using caffeine to wake them up, they use their own stress response from checking their wearable to serve the same purpose. The second are the biometric zombies. They are numb to the score. They are only checking out of habit. They stopped caring a long time ago.

Funny enough, each of these is headed down the same path. That path is a path of inaction. Inaction from either freezing due to anxiety over what that daily HRV score means for them from a health and wellness perspective or inaction from them never encoding the information in the first place. To my knowledge, I do not believe zombies retain information…but feel free to correct me if this has been disproven.

But to put the absolute cherry on top of an already perplexing dilemma, the number that most people see in the morning is only a fraction of what matters. So, even if they chose to act, they would likely act on the wrong information.

“Okay Dr. Jay, what’s the point?”

The point is, you’re wasting your precious time with both inaction and action based on these metrics that you view daily, and there is a better number sitting right next to the one you have been staring at. Well…the number is actually found in the app I built for my clients, probably not the one you are using.

What You Got Wrong…and How to Fix It

You may have interpreted what I have written thus far to be me advocating for you to stop checking your biometric data each day. Well, that might actually be more true than not. Actually, a great example of this is your stock portfolio. If you check this on a daily basis without having an understanding of both the ebbs and flows of the market and what any of the numbers actually mean, you will drive yourself wild. This happens all the time with even the most seasoned of investors. Example: quarterly reports are provided by organizations that give less than exciting results, and BOOM a sell-off occurs. People make rash decisions quickly due to anxiety and panic.

Interpreting your nervous system data is no different.

You review your HRV score in the morning, see it is lower than normal, and immediately make a rash decision.

“Well, guess I shouldn’t train today.”

“Guess I am stressed.”

“Today is going to be a bad day.”

All of this was based on a poor interpretation of a data point. To be fair, this is not all your fault. Most wearable companies provide absolute nonsense in the form of proprietary scores that can either give you a false sense of hopefulness or helplessness. You see the red on the screen and therefore you interpret it as “bad.”

NET NET: You made a rash decision (inaction or action) based on a singular reading that you don’t understand. Oh…and you did so without addressing any context. Don’t worry…you are not alone.

What You Should Be Looking At

If looking at the morning number is not the right solution, then what is?

That number you look at is typically going to be RMSSD, which is the Root Mean Square of Successive Differences. Yes, that is a mouthful, but it is essentially the beat-to-beat variation in your heart rhythm, and it is the cleanest window we have into parasympathetic activity. It is a validated metric for measuring “vagally-mediated” nervous system change.

Measure it once…doesn’t tell us much. Measure it every day for three months? Now we have true nervous system data to play with! But even two weeks’ worth of data can start to tell us something about where your nervous system is trending.

The reason for not simply looking at RMSSD and making a broad interpretation is that one reading (one day) can change significantly in a very short period of time (e.g., test 10 minutes later and it could be half of what it was, depending on context).

Instead, what if we still collected and analyzed RMSSD, but we did it over the span of larger timeframes instead of just a single day? Now we are cooking!

This is where another metric comes into play, and it is the primary metric I review and build plans around for all of my clients. It’s called HRV-CV.

Request access to the appThe number you have been staring at, and the number sitting right next to it. Both from this morning’s reading in the Resilient Engine app.

What is HRV-CV?

Put technically, HRV-CV is the coefficient of variation of heart rate variability. Even more technically, you take the standard deviation of your HRV readings across a 7-day window and divide it by the mean of those readings. Then you get a number that is a percentage. The percentage is not telling you how high your HRV is. Instead, it is telling you how much it moves. So, is your HRV holding together (good)? Then the percentage is lower (e.g., 0-8%). Is your HRV pretty variable (not so good)? Then the percentage is higher (e.g., 10-15+%).

Put very simply, HRV-CV looks at how stable your HRV is over a 7-day rolling window. Put even more simply, it measures how stable your nervous system is by watching how much change is occurring in day-to-day readings.

Another technical point: in the Resilient Engine app I use with my coaching clients, I am deriving this from RMSSD and lnRMSSD.

So what?

This should be the number you are reviewing to better inform behavioral decision-making. Not your single day RMSSD reading.

Painting a Clearer Picture of HRV-CV

Here is a little live action of how this data is analyzed and I’ll actually use my own data here.

Here is a week of morning RMSSD readings from a week ago (all in milliseconds): 62, 58, 64, 60, 63, 59, 61.

You can eyeball these readings and see that they hold together fairly tightly. In other words, they kind of look the same. If you have been tracking with me so far, you will likely, and rightly, guess that the HRV-CV (stability) will be low (again…a good thing).

HRV-CV for that week: 3.5%

Here is a week of morning RMSSD readings from this week (all in milliseconds): 78, 45, 68, 39, 72, 51, 74.

Again, eyeballing these, you can see higher highs and lower lows. Certainly, you can see more variance/differences between the daily readings.

But here is the crazy thing!

My average HRV is exactly the same as last week: 61 ms. But the HRV-CV is now 25.6%.

Two weeks of my own morning readings. The mean line is in exactly the same place in both panels. The shaded band is one standard deviation, and it is the whole story.

Have I made my case for this metric now?

You would see the same average for weeks 1 and 2, but in week 2, my nervous system was much less stable. So, my behavioral decision-making changes significantly.

HRV-CV Decision-Making 101

Now that, hopefully, you are convinced that this is a viable metric to use, let’s talk about how to use it.

Let me be very clear.

This is still an ever-growing field of study and there is not currently an agreed-upon framework for decision-making that is backed by numerous studies. Instead, there is a soft framework based upon the limited data we have and the work of top researchers in the field (mostly coming from using this metric in exercise science and physiology).

My current use for myself and with clients:

• Stable: 0-8%

• Caution: 8-15%

• Compromised: 15+%

These are standards that come mostly from the top voices in exercise science and physiology.

There was a study performed by Grosicki and colleagues (2026), which looked at HRV-CV as it relates to behavior, noting behaviors like poor sleep, alcohol, and sedentary lifestyle all significantly impacting nervous system stability. Nothing game-changing here, but an interesting study to check out. This is coming from night-time HRV readings from wearables. There is a whole debate here, as I actually favor morning readings over nocturnal. For more expansive takes on this, check out Marco Altini’s Substack for his take. (Altini is one of the co-authors on that study.)

Another general rule of thumb: HRV-CV tracking for one week is less robust than one month, which is less robust than three months, and so on.

However, decision-making based on these numbers is more defensible than average RMSSD over the course of the week, and far more defensible than singular RMSSD readings.

My general rule of thumb, even with everything I have already stated regarding objective data, is that objective data alone is a poor proxy for making behavioral decisions. It can greatly inform your decision, but how you feel is equally, if not more, important.

You should be checking your subjective experience before making decisions. I have my clients do this before they check data.

If we are to make decisions based on the percentage classifications I mentioned, it is normally for dialing up or down the intensity of the day, whether this is focusing on exercise, stress resiliency, sleep, or nutrition. It is rare that our nervous system is just so taxed that we should “not do anything.” Even when CV is up at 20-30%. But it sure is a sign to do something different.

I always come back to the fundamentals and tell my clients that if HRV-CV is in the caution or compromised zone, it is more a matter of modifying behaviors and placing more emphasis on positive behaviors than of eliminating things.

Conclusion

Want the number itself?

HRV-CV is the metric the Resilient Engine app is built around. A two minute morning reading from your phone camera, and you get the 7-day stability number rather than a proprietary score you cannot interpret.

Request access to the app

Want someone to read it with you?

I work one to one with athletes, executives, and operators at Thrive Wellness and Performance, where this data drives an actual protocol rather than a daily mood.

Work with me directly

To wrap this up, put simply, stop letting HRV ruin your day by looking at and interpreting the wrong thing. Get out of the pattern of inaction and move towards useful metrics like HRV-CV.

References

Grosicki, G. J., et al. (2026). Heart rate variability coefficient of variation during sleep as a digital biomarker that reflects behavior and varies by age and sex. American Journal of Physiology-Heart and Circulatory Physiology. https://doi.org/10.1152/ajpheart.00738.2025

Dr. Jay Wiles is a clinical health and performance psychologist, Chief Health and Performance Officer at Ohm Health, BCIA-certified in peripheral and HRV biofeedback, and the originator of Nervous System Resiliency Training (NSRT). He works with athletes across the NHL, NFL, MLB, PGA, and Formula 1, alongside executives and operators in high-performance environments.

There is a physiological capacity that determines how well you sleep, how quickly you recover, how steadily you handle pressure, and how fluidly you move between effort and rest. It does not have a common name. Most people have never trained it directly.

It is the capacity of your autonomic nervous system to shift states on demand. To move into focus when focus is required. To downshift into recovery when the work is done. To mount an appropriate stress response when stress arrives, and to return to baseline when it passes. I call this capacity autonomic flexibility, and in more than a decade of clinical practice with athletes, executives, and operators, I have come to believe it is the most under-trained system in human performance.

This publication is about training it.

The framework I use to train it is called nervous system resiliency training, or NSRT. It is a structured, protocol-driven discipline that I have developed, grounded in two decades of research in psychophysiology, applied behavioral medicine, and HRV biofeedback. It is not a technique. It is not a tradition. It is not a hack. It is closest, in spirit and structure, to how exercise physiology thinks about cardiovascular training. There is a target adaptation, a primary stimulus, a measurement system, and a progression logic.

This first piece is the architectural map. Future pieces will go deep on each component. But before any of that work makes sense, the shape of the framework has to be clear.

The Adaptation: Autonomic Flexibility

Every training discipline is defined first by what it adapts. Strength training adapts the neuromuscular system to produce force. Cardiovascular training adapts the cardiopulmonary system to sustain output. Nervous system resiliency training adapts the autonomic nervous system to shift states.

Autonomic flexibility is not a mood. It is not a mindset. It is a measurable physiological capacity, expressed in the speed and precision with which your nervous system can move between sympathetic activation and parasympathetic recovery. The literature on heart rate variability, baroreflex sensitivity, and respiratory sinus arrhythmia has been quantifying this capacity for decades (Lehrer & Gevirtz, 2014; Shaffer & Ginsberg, 2017).

What does autonomic flexibility look like in a life? Falling asleep within minutes of getting into bed. Shifting from a difficult conversation into a focused work block without lingering activation. Recovering from a hard training session faster than you used to. Remaining steady during high-pressure moments that would previously have knocked you sideways. Returning to baseline after stress without rumination.

These are physiological outcomes, not psychological achievements. And like any physiological capacity, autonomic flexibility responds to structured training.

The Architecture

NSRT is built on three structural elements: a substrate and two training axes. The substrate is sleep. The two training axes are bottom-up and top-down. Everything else in the framework lives somewhere on this map.

Sleep is the substrate. It is not another health pillar or an equal partner. It is the physiological foundation on which any nervous system training is built. You cannot train autonomic flexibility in a chronically sleep-deprived nervous system, in the same way you cannot build strength in a chronically malnourished body. Sleep does not need to be perfect for training to work, but it needs to be sufficient. The training adapts what sleep makes available.

The bottom-up axis trains the autonomic nervous system through targeted physiological stimuli. The body is the input. The primary stimulus is resonance frequency breathing, a specific paced-breathing protocol, practiced at the individual’s unique resonance rate, that produces large coherent oscillations in heart rate and blood pressure. Over weeks and months, repeated exposure to this state strengthens the entire autonomic regulatory apparatus (Lehrer et al., 2003; Vaschillo et al., 2006). Other physiological stimuli sit on this axis as secondary tools, used to extend or support the primary work, but resonance frequency breathing is the foundation.

The top-down axis trains the autonomic nervous system through attention. The mind is the input. The primary practice on this axis is what I call mental attunement training, or MAT: the deliberate practice of attending to internal physiological and emotional state with the specific aim of informing autonomic regulation. MAT is distinct from mindfulness as it is commonly taught. Mindfulness, in most of its forms, cultivates a quality of awareness as an end in itself. MAT cultivates awareness as the lever for autonomic change. The lineage is real and worth naming, MAT draws from the contemplative traditions formalized in Mindfulness-Based Stress Reduction (MBSR) by Jon Kabat-Zinn, from Buddhist practice that long preceded it, and from the present-moment contact work of acceptance and commitment therapy. But the target is different. MAT is mindfulness sharpened to a physiological purpose.

These two axes work together, but they enter in sequence. Bottom-up first. The capacity to attend usefully to your internal state depends on having an internal state regulated enough to be worth attending to. Trying to do top-down work in a chronically dysregulated nervous system is like trying to read in a moving car. The bottom-up axis builds the stability. The top-down axis trains what to do with it.

What This Is Not

Four adjacent categories get confused with NSRT often enough that it is worth being specific about each one.

NSRT is not breathwork. Breathwork is a large umbrella that includes Wim Hof, holotropic breathing, box breathing, Pranayama traditions, and countless other practices. Most are taught as techniques for acute state change. Some are useful. None are built around resonance frequency as the primary training stimulus, and none track adaptation across weeks and months. NSRT uses breath the way strength training uses a barbell, as the instrument through which a measurable capacity is built over time.

NSRT is not meditation. Meditation, in most forms, is a practice of attention with mental cultivation as the primary target. NSRT inverts that, the autonomic nervous system is the primary target, even when the mental effects are real. (MAT, the top-down primary, is a closer cousin to certain meditation traditions, but its target and its measurement are different.)

NSRT is not vagus nerve hacking. Cold face plunges, humming, gargling, and ear-point stimulation have produced an entire cottage industry. Some have modest empirical support. Most do not. The deeper problem is that isolated vagal stimulation does not produce the sustained training adaptations that resonance frequency practice produces. Vagus nerve hacking treats the vagus as a switch to flip. NSRT treats the autonomic nervous system as a capacity to build.

NSRT is not stress management. Stress management is a psychological framing that focuses on cognitive reappraisal and behavioral coping. These are useful tools, and I use them in clinical practice. But they are downstream of the underlying physiology. NSRT targets the substrate. Stress management works on top of whatever substrate happens to be there.

Why This Matters Now

The last decade of wearable technology has taught an enormous number of people to pay attention to their nervous systems for the first time. That is a real contribution. HRV, once a niche metric used in academic cardiology and psychophysiology labs, is now displayed on millions of wrists and fingers every morning.

But something went wrong in the translation from the lab to the consumer.

The research that established HRV as a meaningful measurement was, almost without exception, research on interventions. Lehrer, Gevirtz, and the Vaschillos did not build the foundation of HRV biofeedback by showing that people who measure their HRV do better than people who do not. They built it by showing that people who train at their individual resonance frequency, for structured durations, over weeks and months, produce specific adaptations in baroreflex gain, vagal tone, anxiety, sleep, and emotional regulation (Lehrer & Gevirtz, 2014; Vaschillo et al., 2006).

The consumer wearable industry took the measurement and left the intervention behind.

The result is a generation of users who know their HRV score, watch it fluctuate, feel good when it goes up, feel bad when it goes down, and have no protocol for changing it beyond vague recommendations about sleep and stress. Tracking has replaced training. The industry has, largely, allowed this to happen because tracking is easier to sell.

Reading the gauge is not training the engine.

NSRT is the correction.

What This Substack Will Be

The Resilient Engine is the place where I will build out this framework in public, one piece at a time. Future writings will go deep on each component named here: resonance frequency breathing as the bottom-up primary, MAT as the top-down primary, sleep as the substrate, the secondaries on each axis, the assessment framework, the protocol structure, the dosing logic, the progression model, the individualization layer, the timeline of training effects.

Some pieces will be technical. Others will be argumentative. The conversation about how to train the nervous system has been dominated by the wearables industry on one side and the wellness industry on the other, and both have left the actual training discipline largely unbuilt. The work here is to build it.

If you have been tracking your nervous system for years without a protocol for changing it, this publication is for you. If you have tried breathwork and found it useful but vague, this publication is for you. If you are a clinician or coach looking for a structured way to think about autonomic training with the people you work with, this publication is for you.

The nervous system is the engine of human performance. It is also the system almost no one has been taught to train.

That changes here.

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References

Lehrer, P. M., & Gevirtz, R. (2014). Heart rate variability biofeedback: How and why does it work? Frontiers in Psychology, 5, 756.

Lehrer, P. M., Vaschillo, E., Vaschillo, B., Lu, S. E., Eckberg, D. L., Edelberg, R., Shih, W. J., Lin, Y., Kuusela, T. A., Tahvanainen, K. U. O., & Hamer, R. M. (2003). Heart rate variability biofeedback increases baroreflex gain and peak expiratory flow. Psychosomatic Medicine, 65(5), 796–805.

Shaffer, F., & Ginsberg, J. P. (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health, 5, 258.

Vaschillo, E. G., Vaschillo, B., & Lehrer, P. M. (2006). Characteristics of resonance in heart rate variability stimulated by biofeedback. Applied Psychophysiology and Biofeedback, 31(2), 129–142.

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Dr. Jay Wiles is a clinical health and performance psychologist, the Chief Health and Performance Officer at Ohm Health, BCIA-certified in peripheral and HRV biofeedback, and the originator of Nervous System Resiliency Training (NSRT). He works with athletes across the NHL, NFL, MLB, PGA, and Formula 1, alongside executives and operators in high-performance environments