Understanding heart rate variability in the age of wearables

The Rise of HRV Awareness: What Are We Actually Measuring?

With the proliferation of wearable devices like Oura Ring, WHOOP, and Apple Watch, Heart Rate Variability (HRV) has become a household term in the health and wellness community. But what exactly is HRV, and what do these numbers on your wrist really mean?

What is HRV?

Heart Rate Variability measures the variation in time intervals between consecutive heartbeats. Contrary to what many believe, a healthy heart doesn't beat like a metronome - instead, it constantly adjusts the intervals between beats in response to breathing, stress, physical activity, and various physiological demands. This variability is actually a sign of a healthy, responsive autonomic nervous system.

A Natural Example

Imagine walking through a peaceful forest. High HRV means your heart responds dynamically to your environment: the gentle rustling of wind through the trees allows your heart rate to slow and become more relaxed. Suddenly, a small animal darts across your path — your heart rate quickly accelerates in response. Then, as you realize there's no threat, it smoothly decelerates again. This ability to rapidly adapt and recover — speeding up when needed, slowing down when safe — reflects a responsive and resilient cardiovascular system. High HRV indicates your heart can dance with your environment rather than marching rigidly through it.

Time-Domain Metrics: What Your Wearable Shows You

Most consumer wearables report time-domain HRV metrics, with RMSSD (Root Mean Square of Successive Differences) being the most common. RMSSD specifically reflects the beat-to-beat variations in heart rate and is considered a reliable indicator of parasympathetic (rest-and-digest) nervous system activity. Other time-domain metrics include:

• SDNN (Standard Deviation of NN intervals): Reflects overall HRV and both sympathetic and parasympathetic activity
• pNN50: The percentage of successive intervals that differ by more than 50ms, another marker of parasympathetic activity

Frequency-Domain Metrics: The Hidden Layer

Beyond what most wearables display, HRV can also be analyzed in the frequency domain, revealing different aspects of autonomic function:

• HF (High Frequency, 0.15-0.4 Hz): Primarily reflects parasympathetic activity and respiratory influences
• LF (Low Frequency, 0.04-0.15 Hz): Reflects both sympathetic and parasympathetic modulation
• VLF (Very Low Frequency, 0.003-0.04 Hz): Associated with longer-term regulatory mechanisms, including hormonal and thermoregulatory influences
• LF/HF Ratio: Often interpreted as a measure of sympatho-vagal balance, though this interpretation remains debated in the scientific community

These frequency-domain measures provide a more nuanced picture of how different branches of your autonomic nervous system are functioning and interacting. In the field of HRV research, there are also other calculation methods besides time-domain and frequency-domain analysis.

Is Higher HRV Always Better?

The common wisdom in the wellness community is simple: higher HRV equals better health. But is it that straightforward?

The General Truth

In general, yes - higher HRV is associated with:

• Better cardiovascular fitness
• Greater stress resilience
• Improved emotional regulation
• Lower risk of cardiovascular disease
• Enhanced recovery capacity
• Increased longevity

Research has consistently shown that individuals with higher HRV tend to have better overall health outcomes and lower mortality risk across various age groups.

The Nuance

However, the relationship isn't always linear. Context matters enormously:

• Baseline matters: An HRV of 40 ms might be excellent for a 60-year-old but concerning for a 25-year-old athlete
• Trends matter more than single values: A consistently declining HRV trend may be more significant than one low reading
• Extremely high HRV isn't always ideal: In some cases, excessively high HRV can indicate overtraining or certain medical conditions
• Individual variation is huge: What's "normal" varies dramatically between individuals based on age, fitness level, genetics, and lifestyle

The key is understanding your personal baseline and monitoring trends over time rather than fixating on absolute numbers or comparing yourself to others.

HRV: The Window into Your Vagus Nerve, Health, Stress, and Sleep

The Vagus Nerve Connection

HRV is fundamentally a reflection of vagal tone - the activity level of your vagus nerve, the longest cranial nerve in your body. The vagus nerve is the primary conduit of the parasympathetic nervous system, often called the "rest and digest" system. When your vagus nerve is functioning optimally (high vagal tone), it can quickly modulate your heart rate in response to various stimuli, resulting in higher HRV.

HRV and Overall Health

Your HRV serves as a real-time biomarker of your body's ability to adapt to stress and maintain homeostasis. It reflects:

• Autonomic balance: The interplay between your sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous systems
• Metabolic health: HRV correlates with insulin sensitivity and metabolic flexibility
• Inflammatory status: Lower HRV is associated with chronic inflammation
• Cardiovascular health: HRV predicts cardiovascular events and mortality

HRV and Stress

Chronic stress is one of the most consistent suppressors of HRV. When you're under stress:

• Your sympathetic nervous system dominates
• Vagal activity decreases
• HRV drops
• Your body loses its adaptive flexibility

Conversely, stress management techniques like meditation, deep breathing, and vagus nerve stimulation can enhance HRV by shifting the balance back toward parasympathetic dominance.

HRV and Sleep

The relationship between HRV and sleep is bidirectional:

• Sleep quality affects HRV: Poor sleep suppresses HRV, while restorative sleep enhances it
• HRV predicts sleep quality: Lower evening HRV often predicts poorer sleep quality
• Sleep stages show different HRV patterns: HRV typically increases during deep sleep (reflecting high parasympathetic activity) and decreases during REM sleep

Nighttime HRV, particularly during the first sleep cycles, is considered one of the most reliable HRV measurements because it's less influenced by external factors like physical activity, meals, and stress.

HRV and Longevity

Perhaps most compellingly, HRV has emerged as a potential predictor of longevity. 

• Lower all-cause mortality
• Reduced cardiovascular disease risk
• Better cognitive function in aging
• Enhanced resilience to age-related decline

In essence, HRV serves as a biomarker of biological age - how well your body is aging regardless of your chronological age.

Can taVNS Reliably Boost HRV? Setting Realistic Expectations

Given the clear connection between vagus nerve activity and HRV, it seems logical that transcutaneous auricular vagus nerve stimulation (taVNS) should consistently boost HRV. However, the reality is more complex.

Individual Response Variability

Research, including our own studies, reveals significant individual differences in HRV response to taVNS:

• Baseline HRV matters: Individuals with already high baseline HRV tend to show smaller improvements, possibly due to a ceiling effect
• Stimulation parameters: Different individuals may respond optimally to different stimulation intensities, frequencies, and durations
• Physiological state: Your body's response to taVNS varies depending on your current state - whether you're stressed, relaxed, fasted, post-meal, sitting, or lying down
• Time of day: Circadian rhythms influence autonomic function and thus HRV response to stimulation

The Sensitivity Challenge

HRV is remarkably sensitive to momentary changes:

• A single deep breath can temporarily increase HRV
• Standing up can immediately decrease it
• A stressful thought can suppress it within seconds

While taVNS can produce acute increases in HRV during or immediately after a session, achieving sustained improvements in average daily or nighttime HRV over weeks and months is considerably more challenging.

Our Research Findings

In our reproducibility study with 60 healthy university students, we examined HRV responses to identical taVNS parameters across two sessions separated by one week:

• Over 60% of participants showed similar HRV enhancement responses to the same taVNS parameters in both sessions
• However, approximately 40% exhibited completely different responses in the second session compared to the first

This highlights both the promise and the challenge: while many people show consistent positive responses, a substantial portion demonstrate variable responses that standard, one-size-fits-all parameters cannot adequately address.

What Should We Expect from taVNS and HRV?

Setting realistic expectations is crucial:

1. Acute effects are more reliable: Many users will experience HRV increases during or immediately after taVNS sessions
2. Long-term trends require consistency: Sustained improvements in baseline HRV typically require regular, ongoing use over weeks to months
3. Individual optimization is key: What works for one person may not work for another
4. Holistic approach matters: taVNS works best as part of a comprehensive approach including sleep hygiene, stress management, and healthy lifestyle habits

The Future: Personalized, Closed-Loop taVNS

The challenge of individual variability is precisely why we're developing next-generation solutions. Our future product ecosystem will:

• Integrate third-party HRV data from devices like Oura Ring, WHOOP, and Apple Watch
• Incorporate real-time HRV sensing through our own sensors
• Assess pre- and post-session changes to understand your individual response patterns
• Adjust stimulation parameters dynamically based on your real-time physiological feedback
• Track long-term trends to optimize protocols over weeks and months

This closed-loop approach represents a shift from generic stimulation to truly personalized neuromodulation - finding the specific parameters and timing that work best for YOUR nervous system to achieve YOUR optimal HRV enhancement.

Conclusion: HRV as a Journey, Not a Destination

Understanding HRV is understanding the language your body uses to communicate its state of balance, resilience, and health. While higher HRV generally indicates better health and longevity potential, the path to improving it is highly individual.

taVNS offers a promising, non-invasive tool for enhancing vagal tone and HRV, but it's not a magic bullet. The future lies in personalized approaches that adapt to your unique physiology, lifestyle, and goals.

At ZenoWell, we're committed to advancing both the science and the technology to make personalized vagus nerve stimulation a reality - helping you not just track your HRV, but actively optimize it for better health, resilience, and longevity.

Reference:

1. Yelisyeyeva, O., Kaminskyy, D., Semen, M., Chelpanova, I., & Semen, K. O. (2025). Redox Metabolism and Autonomic Regulation During Aging: Can Heart Rate Variability Be Used to Monitor Healthy Longevity?. Biomedicines, 13(1), 161.
2. Gianlorenço, A. C., Pacheco-Barrios, K., Daibes, M., Camargo, L., Choi, H., Song, J. J., & Fregni, F. (2024). Age as an effect modifier of the effects of transcutaneous auricular vagus nerve stimulation (taVNS) on heart rate variability in healthy subjects. Journal of Clinical Medicine, 13(14), 4267.
3. Percin, A., Ozden, A. V., Yenisehir, S., Pehlivanoglu, B. E., & Yılmaz, R. C. (2026). Does Baseline Autonomic Nervous System Activity Affect the Outcomes of Transcutaneous Auricular Vagus Nerve Stimulation?. Archives of Medical Research, 57(3), 103324.
4. Xiao et al., Reproducibility of HRV as a Biomarker for TranscutaneousAuricular Vagus NerveStimulation, Organization for Human Brain Mapping, 2025, Brisbane.