Author’s Note: The initial inspiration for this article, as well as the research data and theories referenced throughout, comes primarily from my instructor Dr. Sylvain Laborde at the German Sport University Cologne. He has long been active on the front lines of HRV research and application. My sincere thanks to Dr. Laborde.

The author is a current master’s student in the Department of Sport Psychology at the German Sport University Cologne (Deutsche Sporthochschule Köln). The author has no financial ties to any researchers, devices, or app providers mentioned in this article.

Introduction: Starting With Questions You May Not Know You Have

What is HRV?

You might know that HRV stands for Heart Rate Variability, and that its Chinese translation is “心率变异性.” You may know that your wearable device can measure HRV; that the human heart is not a metronome; and that HRV represents the variation in time intervals between consecutive heartbeats. You might also know that trends in HRV relate to health or “readiness.”

You may also know that HRV levels vary from person to person and cannot be compared across individuals; and perhaps you’ve heard about how and when HRV should be measured.

But beyond that—how much do you really understand about HRV?

Do you focus only on the HRV value changes shown under “Health” on your device, without knowing the evidence-based meaning behind an increase or decrease in HRV? Do you know that your device claiming to measure HRV might actually be estimating it?

This highly sensitive and constantly fluctuating metric—when exactly should it be measured, and under what conditions? How should it guide lifestyle adjustments, including—but not limited to—exercise? And going a step further, are there specific forms of training that can enhance or improve HRV?

If these questions interest you, then this article (and the follow-up pieces to come) may be well worth your time.

HRV and the Parasympathetic Nervous System: A Reflection of Decision-Making and Adaptive Capacity

In a world full of constant change, we need the ability to regulate our physical and mental states in response to shifting conditions, and to make decisions and take actions that best serve our goals and interests. HRV is an important metric that quantifies this adaptive capacity.

The autonomic nervous system has two branches: the sympathetic nervous system, which acts as the “accelerator,” and the parasympathetic nervous system, which serves as the “brake.” This is common knowledge. But do you know which system is more important for modern-day adaptation?

The answer is the parasympathetic nervous system.

Take heartbeats as an example. Our heartbeat is governed by nerve signals originating from the medulla and realized through the sinoatrial node (SA node) in the heart. The SA node functions as the body’s natural “pacemaker.” Its intrinsic firing rate is 100 beats per minute. In other words, if nothing else intervened, our resting heart rate would naturally be 100 bpm. It is the parasympathetic nervous system—the “brake”—that slows a healthy individual’s heart rate down to 60–80 bpm, or even lower.

With this brake engaged, we maintain a lower heart rate and deeper calm during rest, conserving energy and reserving resources for unexpected changes. Counterintuitively, when faced with a sudden threat, our body does not always activate the sympathetic “fight or flight” response immediately. Such instinctive, primitive reactions can often lead to very poor decisions.

More often—and more adaptively—when encountering a challenge, we first hit the brake and engage the parasympathetic nervous system. Our heart rate slows; our body enters a “freezing” state. In this state, our senses sharpen and our mind becomes more focused, allowing us to make better decisions. Once a decision is made, we release the brake and hit the sympathetic “accelerator” to carry out the most effective action.

A quick side note: some people “freeze” when facing danger. This doesn’t necessarily mean they lack courage. More likely, the danger is too great or too unfamiliar (requiring more processing time), or their ability to smoothly switch between parasympathetic and sympathetic activation is underdeveloped. These limiting factors can and should be trained. There is no need to judge yourself harshly for freezing in fear.

Back to the main topic. The “brake” signal from the parasympathetic nervous system reaches the heart through the tenth cranial nerve—the vagus nerve (C10). Eighty percent of vagal fibers carry information to the brain, while the remaining 20% carry information from the brain to the body. All of this 20% belongs to the parasympathetic branch. Their electrical signals intervene in the firing rhythm and waveform of the sinoatrial node, creating variations—these variations form the basis of the HRV signal.

If the parasympathetic system is functioning well and vagal efferent activity is strong, it sends a stronger “brake” signal. This leads to larger and richer variations in the intervals between heartbeats, which is observed as increased HRV. In such moments, the body spends more time in a state of rest and relaxation.

Conversely, if vagal activity is inhibited—or if sympathetic influence increases while parasympathetic activity weakens—the intervals between heartbeats become more uniform, resulting in decreased HRV. In this state, the body leans more heavily into the “fight or flight” mode—in other words, a heightened state of tension.

When the body is tense and the parasympathetic system is suppressed, decision-making quality and executive function decline. Moreover, people who remain in a tense state for prolonged periods experience elevated inflammation levels. Evidence suggests that vagal afferent fibers can detect increases in inflammatory markers in the body, while vagal efferent fibers can activate the immune system—especially the spleen—through the noradrenergic system, prompting T cells and macrophages to combat inflammation (Bazoukis, Stavrakis & Armoundas, 2023). In other words, the vagus nerve and the parasympathetic system not only act as a “brake” but also as a “firefighter.”

In summary, HRV positively reflects the function of the parasympathetic nervous system and indicates the body–mind system’s adaptive capacity when faced with environmental changes. For this reason, tracking HRV allows us to understand and assess our adaptability and overall “health status” in a broad sense.

Are You Really Measuring HRV? : The Principles Behind HRV Measurement, and How to Choose Devices and Software

There are many consumer electronic products on the market claiming to measure HRV—smartwatches, fitness bands, smart rings, even smart mattresses. Focusing solely on HRV measurement, how reliable are these devices, and how much of what you’re paying is essentially a “tech tax”? To answer this, we need to begin with the basic principles behind HRV measurement.

The term HRV itself is somewhat misleading, because HRV does not measure variation between two “heartbeats.” Instead, it measures variation between two cardiac cycles—more precisely, between two R-peaks on an electrocardiogram (ECG). A more accurate term might be Heart Period Variability (Quigley et al., 2024). But since “HRV” is already widely used, we’ll stick with it for convenience. The key point is: HRV measurement fundamentally depends on collecting ECG data. Therefore, only devices that can record and analyze ECG waveforms can truly measure HRV.

The most typical ECG monitoring devices are the multi-lead systems used in hospitals and laboratories. Clinical HRV monitoring often relies on such multi-lead setups, collecting long, continuous ECG recordings and calculating average interval variation between R-peaks. This method produces the gold-standard HRV data. Consumer wearables cannot provide multi-lead ECG recordings and cannot operate continuously for 24 hours. Whether it’s an Apple Watch, Garmin band, or Oura Ring, they all rely on a compromise solution.

A common method they use is photoplethysmography (PPG). Devices using PPG emit green light into the skin to detect periodic changes in blood flow (pulse). They then infer HRV by analyzing the variability of pulse intervals—what we might more accurately call “PRV” (Pulse Rate Variability). Next time you open the HRV or heart rate feature on your smartwatch or fitness band, lift the device and check whether the back is flashing green. In short: your device does not record ECG data—not even close. It measures pulse changes and then uses algorithms to approximate HRV.

Different manufacturers use different sensors with varying quality and parameters. Cheaper or smaller sensors may be inaccurate to begin with. PPG accuracy depends heavily on stable contact with the skin, meaning that in non-resting conditions—especially during movement—accuracy drops significantly due to micro-slips between the device and the skin. Combined with HRV’s inherent sensitivity to measurement conditions (discussed in the next section), HRV “averages” that mix rest and activity data can be highly inaccurate and misleading. Additionally, different brands use different sensor models and algorithms, making HRV data across devices incomparable. Stop comparing your Apple Watch HRV with your friend’s Oura Ring—it simply doesn’t make sense.

Perhaps for these reasons, major HRV software platforms do not import data from many watches, bands, or rings—because these devices do not measure HRV, and their estimates aren’t reliable enough. As technology improves, consumer devices using PPG may eventually achieve HRV inference with high enough accuracy. But aside from quality and algorithm guarantees, they will require extensive ECG comparison data to validate their accuracy. Until such evidence exists, I cannot recommend using these devices to monitor your HRV. Wear them for fun, check heart rate, tell the time—that’s all fine. But that so-called HRV number? Don’t take it too seriously.

Among consumer-grade electronic devices, the most reliable tools for actually measuring HRV—with acceptable accuracy—are chest-strap heart rate monitors. The most common model on the market is the Polar H10. This device is supported by a wide range of measurement and analysis apps, and it is the preferred wearable HRV device among researchers at the German Sport University Cologne. The H10 consists of a detachable sensor and an elastic chest strap. To use it, simply snap the sensor onto the strap and fasten the strap firmly around the lower chest.

I currently pair the H10 with two main apps: HRV4Training and Elite HRV. HRV4Training allows you to measure and store one HRV reading per day (if you measure again, the new data replaces the old). Combined with a post-measurement questionnaire, HRV4Training helps track correlations between HRV and daily habits such as exercise, alcohol intake, and sleep, as well as the long-term trends in your HRV.

If you need continuous HRV measurement over a specific time window within the day—such as during and after a workout—Elite HRV is the tool for the job. It can track your real-time HRV curve throughout the measurement session and generate data logs. Elite HRV data can also be exported into desktop software such as Kubios HRV, allowing you to build and manage detailed “HRV health profiles” for yourself or others, tied to training and lifestyle patterns.

Across various apps (including Apple Health’s “Heart Rate Variability” section), the most commonly used HRV indicator is RMSSD (Root Mean Square of Successive Differences). RMSSD represents the average variance in time between consecutive heartbeats over a given period, measured in milliseconds (ms). Because it is less affected by breathing and is time-based, RMSSD reflects vagal influence on heart rate quite intuitively. The higher your RMSSD, the higher your HRV—and the stronger your body’s adaptive capacity.

Although we shouldn’t (and don’t need to) compare ourselves to others, it’s hard to resist the urge when we see our own RMSSD value. HRV4Training provides normative data from all its users, separated by age groups. If you collect your HRV using a chest strap or ECG device, you can compare your readings to these norms. Kubios HRV automatically calculates your “parasympathetic recovery index” and “sympathetic stress index” from the imported data and compares them with its built-in norms. There is currently no nationwide HRV normative sample for China. However, a 2018 Dutch study collected RMSSD averages from 13,943 participants (van den Berg et al., 2018). You can check how your adaptive capacity compares to Dutch individuals in your age group.

HRV as a Resource: The Vagal Tank Theory and the “3Rs”

HRV does not directly reflect how much stress we are under, nor does it correspond in a strict one-to-one manner with indicators such as cortisol levels or gut microbiota composition. HRV—especially RMSSD—reflects the functional state of the parasympathetic nervous system and the availability and condition of our resources for coping with stress.

With this understanding, we can naturally think of HRV as an internal “battery” or “energy bar.”
When we are relaxed, the energy tank is full. As life events unfold, the tank drains bit by bit. When it runs low, we experience physical and mental fatigue, increased subjective stress, and diminished self-regulation. If the tank fully depletes, the body and mind may “shut down,” much like a smartphone that suddenly turns off with no battery.

This is precisely the core of Laborde’s Vagal Tank Theory (Laborde, Mosley & Mertgen, 2018b). The theory revolves around the 3Rs. The first R, Resting, refers to our baseline parasympathetic activity—the “full tank” state. The stronger and healthier you are, the higher your baseline HRV (explained below), the higher your upper limit for resource capacity, and the better your ability to deal with stress.

The second R, Reactivity, describes how we expend our tank when responding to challenges in life. Your challenge might be a difficult math problem, a fight with a friend, or a literal pile of bricks. In such moments, the vagus nerve “opens the valve,” draining the tank (“consuming blue”), allowing you to mobilize physical and mental resources to solve the problem.

Once the challenge ends, we enter the third R, Recovery, where we replenish our energy tank. Some people recover poorly or use ineffective methods—their bodies behave like an outdated phone battery that never charges fully, or worse, becomes swollen. Others recover efficiently or use the right strategies, allowing the tank to refill quickly, sometimes even exceeding baseline—raising the upper capacity of the tank.

The 3Rs vividly illustrate why we must resist burnout culture, reject 996 schedules, and avoid endless self-exploitation. If we constantly drain our tank in the name of productivity but fail to replenish it, our bodies and minds will inevitably collapse. Even if we force ourselves to continue, we will become like a phone that never reaches a full charge—always hovering near empty until one day it can no longer charge at all.

I personally experienced such a collapse a little over a month ago. For a week and a half, I attended classes during the day and worked at night, with no rest days and less than six hours of sleep daily. Then I spent Friday to Sunday at the Essen Game Fair, commuting four hours a day, working twelve, and sleeping only four. After returning, I caught a mild cold but didn’t rest; instead, I continued the same schedule for another two weeks. After a month of this, my body finally fell apart—diarrhea, fatigue, low fever, chills, and insomnia. But none of this was due to infection—it was simply collapse from lack of rest. On my worst days, my HRV dropped to nearly single digits (which is borderline “corpse mode”).

Realizing this was unsustainable—and life-threatening—I completely rested for two days and then significantly reduced my workload. After about two weeks of gradual recovery, coinciding with a trip to Japan, my HRV finally returned to—and exceeded—its baseline level.

This is my personal experience and not something to generalize to everyone. I share it to illustrate that our energy tank is not infinite, and the consequences of draining it completely can be severe. To allow the tank to refill, we must grant ourselves adequate rest and recovery. When a battery swells, you can replace the battery—or even the device. But when your mind and body “swell,” sorry to say, with the little money earned through burnout culture, you truly can’t afford to replace anything.

When talking about rest and recovery, the natural questions arise:
What kinds of activities drain us the most?
How can we tell when our energy tank is nearing depletion?
How do we know if we’ve rested “enough”?
Which activities help the tank recover (or even increase) most effectively?

The answers to these questions can all be found through HRV. By measuring and tracking our HRV across time, we can clearly see which activities consume the most energy—and which forms of rest replenish it the fastest.

More Sensitive Than You Think: Measuring and Tracking HRV

That said, you cannot simply buy a wearable device, strap it on, and trust the HRV number it “measures.” Numerous studies (Fatisson, Oswald, & Lalonde, 2016; Laborde, Mosley & Mertgen, 2018a) show that HRV is extremely sensitive. Daily physiological fluctuations—including blood pressure, body temperature, and endocrine activity—affect HRV. Respiration and heart rate especially interfere with HRV measurements. If your device cannot even filter out these sources of noise, its readings are essentially meaningless.

Age, sex, weight, BMI, and genetics (including ethnicity) also influence HRV. For example:

• HRV declines steadily with age.
• Women generally have higher HRV than men in the same age group.
• People with obesity tend to have higher resting heart rates and lower HRV.
• Travel, frequent commuting, or increased training loads tend to lower HRV.
• Illness, alcohol consumption, medications, and circadian rhythms can significantly reduce HRV.

To correctly interpret your HRV, you need to not only measure it but also know what state your body is in at the time of measurement.

But that’s still not the whole picture. HRV is so sensitive that even small changes in your physical state during measurement can cause immediate fluctuations in the recorded values. In class, Laborde once led us through a live demonstration. Several students (myself included) wore heart-rate chest straps and opened Elite HRV to track our HRV in real time. We first lay flat on the floor for two minutes, then quickly stood up, and after holding a standing posture for one minute, quickly sat down again. During those brief moments of “standing up” and “sitting down,” our HRV values shifted dramatically. Afterwards, we switched between different physical activities and breathing patterns—and every switch produced significant changes in HRV.

For this reason, if you want valid HRV data, you must ensure that measurements are taken under the most consistent conditions possible. If you measure HRV at different times of day, in different physical or mental states, or even in different body postures, the readings may become inconsistent or meaningless.

If we want to measure HRV once per day, keep each measurement within 3–5 minutes, and use long-term trends as the key indicator, then the best approach is to measure at a consistent time and in a stable physiological state. The ideal moment is right after waking up.
Here is the HRV measurement routine I currently follow:

1. Alarm rings (wake up)
2. Put on clothes, get out of bed, go to the toilet (morning urination)
3. Wash hands—but do not brush teeth or wash your face yet
4. Return to the bedroom, put on the heart-rate chest strap, sit down, and regulate breathing for one minute
5. Open HRV4Training and measure HRV (default duration: one minute)
6. Fill out the behavior-related questionnaire in HRV4Training
7. Remove the chest strap and continue with morning activities (brushing teeth, washing face, breakfast, etc.)

If using a chest strap is inconvenient, you can use an arm-worn optical band (e.g., Rhythm 24). HRV4Training currently supports reading HRV data from the Apple Health app. However, as noted earlier, because the Apple Watch measures HRV multiple times per day—and wrist-based HRV accuracy during movement is limited—the best approach (if you only have an Apple Watch) is to follow the same morning routine and measure HRV as soon as possible after waking. Alternatively, you can enable continuous overnight HRV tracking so the watch records your HRV throughout sleep.

After collecting at least 4–6 weeks of data, HRV4Training will let you review your HRV trends and gradually understand how your exercise, rest, and lifestyle habits affect your parasympathetic system and overall “adaptive capacity.” With this foundation of data and knowledge, you can begin using HRV to guide your training and undertake targeted practices to improve HRV and adaptability. For details, please see my next article.

Some Key Takeaways (Take-home Message)

If you forget all the technical terms after reading this article, at least take with you the following core understandings about your body’s “energy tank”:

• HRV is the variability of your heartbeat rhythm—and also your capacity to adapt to the world. Don’t treat it as a simple health score, and don’t become anxious by comparing it with others. High HRV means your body has the resources and ability to respond more effectively to challenges—whether that’s a major exam, high-intensity training, or a night of poor sleep. Low HRV is a warning that your “blue bar” is nearing zero, and you should avoid making major decisions or performing demanding tasks in that state.
• Your body needs the brake more than the accelerator. Modern life doesn’t lack stimulation. If something goes wrong with your physical or mental state, it’s unlikely because your “accelerator” isn’t working—but very likely because your “brake” has gone rusty from underuse. Your HRV reflects the energy level of your vagus nerve and parasympathetic nervous system (the brake). This precious braking system not only keeps you calm, but also quietly helps fight inflammation and repair your immune system. Maintaining and improving HRV is essentially maintaining the brake pads that keep you alive.
• Manage your “energy tank” wisely: resting isn’t laziness—it’s strategic action. Remember the Vagal Tank Theory and the 3Rs (Resting, Reactivity, Recovery). Your energy tank is finite. Whenever you must expend significant energy (Reactivity) to face life’s challenges, you must follow up with high-quality recovery. When HRV remains low over time, lying down to rest isn’t weakness—it is the smartest, most necessary tactical adjustment to prevent system failure.
• Reject “junk data”: good decisions require accurate measurements. HRV is extremely sensitive—affected by breathing, posture, mood, and even tiny movements. Not all devices can measure HRV accurately. Don’t overly trust your wristband, smartwatch, or ring—their HRV data may be estimates, not measurements. If you want to use HRV to guide training or understand your body, use more accurate equipment such as a chest-strap monitor, and collect HRV under fixed conditions during a consistent “morning routine.”