If you’ve been reading this journal, you’ve seen me reference HRV (heart rate variability) in almost every essay. Overnight critical meeting, jet lag, sleep, decisions, recovery… HRV keeps coming up because it’s the closest thing we have to a real-time mirror of how the body is operating.
But I realized I’ve never paused to explain what it actually is, why it matters, and how to start using it without falling into the optimization trap.
So let’s pause here. This is the foundational piece. Future essays will rest on this one, including the next one in this series, where I’ll get into why HRV is the upstream variable of decision quality, and why every founder should treat it as a business KPI.
Today, the basics done well :)
HRV is the biomarker that tells you, every morning and during the day when measured, how recovered the system that runs your business and life actually is.
What HRV actually is
Heart Rate Variability is not your heart rate.
Your heart rate is the average of beats per minute. HRV is something different and more revealing: the variation in time between consecutive heartbeats.
Your heart doesn’t beat like a metronome. Even when it averages 60 beats per minute, the actual interval between two consecutive beats is never exactly one second. Sometimes it’s 0.92, sometimes 1.08, sometimes 0.97. That micro-variation, measured in milliseconds, is what we call HRV.
And here’s why that micro-variation matters: it’s a real-time signature of your autonomic nervous system, the system that operates everything you don’t consciously control. Breathing, digestion, hormonal rhythms, recovery, the response to stress.
The autonomic nervous system has two branches:
The sympathetic: activation, alertness, fight-or-flight. The branch that mobilizes you when there’s something to deal with.
The parasympathetic: rest, repair, digestion. The branch that restores you when there isn’t.
When both branches are well coordinated and your body is recovered and adaptable, your HRV is high. The variation between heartbeats is broad. Your system has range, it can shift gears between activation and recovery, between focus and rest, between push and pause.
When the system is in chronic alert, exhausted, or operating under unmetabolized stress, HRV drops. The variation narrows. The heart starts to beat more like a metronome, and a metronomic heart is a stuck system. Stuck in one mode, usually sympathetic. Reactive instead of responsive.
The most recent reviews of the field (Liu et al. 2025, Wang et al. 2025) confirm what’s been emerging for the last decade: HRV is one of the most reliable non-invasive biomarkers we have for autonomic function and adaptive capacity.
In plain language: it tells you whether your nervous system is in a state of range, or in a state of constraint.
A bit more depth (for those who want it
For readers with a clinical or scientific background, a few additional layers worth naming:
The variation HRV measures is called R-R interval variability (or N-N interval, when artifact-corrected). It reflects, primarily, vagal tone, the activity of the vagus nerve, the main carrier of parasympathetic signaling between the brainstem and the heart.
Several metrics derive from this:
RMSSD (root mean square of successive differences): the most commonly reported short-term HRV metric in consumer wearables. Reflects parasympathetic activity.
SDNN (standard deviation of N-N intervals): captures total HRV across both branches, typically over longer windows.
LF/HF ratio: frequency-domain analysis sometimes used to estimate sympathetic-parasympathetic balance, though its interpretation is debated in the recent literature.
For practical use, most consumer wearables report some version of RMSSD calculated overnight. That’s the number you see in your Oura, Ultrahuman, Whoop, or Apple Watch app each morning. It’s a good proxy for vagal tone during sleep — when the system is least disturbed by external inputs.
If you don’t have a clinical background, you don’t need to memorize any of this. You just need to know that the number on your screen is an estimate of how recovered your autonomic nervous system was during the previous night.
Why this matters in real life
Most of us track outputs, what we did, what we delivered, what we shipped. Almost no one tracks the state in which we did it.
HRV gives us continuous, daily feedback on three things that no other metric captures:
Recovery readiness. How prepared is your system today to handle real demand — physical, cognitive, emotional? A high morning HRV tells you you’re recovered. A suppressed HRV tells you yesterday left a debt — and pushing through today will deepen it.
Cumulative load. Are you accumulating systemic stress, or are you actually metabolizing it? An HRV that trends downward over weeks isn’t a bad day, it’s a signal that the system is in chronic overload, even if subjectively you “feel fine.”
The real cost of specific inputs. Alcohol last night. A 3 am call. Missed sleep. Late dinner. A heavy travel day. All of those show up the next morning in your HRV. The data stops being abstract, it becomes operational.
This last one is what changed everything for me. I stopped guessing what was draining my system and started seeing the cost on the screen.
What HRV is not
This is important because the optimization culture has filled the internet with bad takes about HRV.
HRV is not a “high = good, low = bad” metric. What matters isn’t the absolute number it’s your baseline and your deviation from it. Someone whose normal HRV is 45 ms and lives steadily there is in better autonomic balance than someone with a normal of 95 ms who suddenly drops to 60.
There is no universal “optimal HRV.” Your range depends on age, sex, genetics, fitness level, training history. Comparing your HRV against a friend’s, or against an internet average, is meaningless.
More HRV is not always better. Extremely high HRV sustained over time can also signal autonomic dysregulation, particularly in athletes overtraining or in people with certain cardiac conditions. The goal isn’t to maximize HRV. It’s to know your baseline, recognize your patterns, and respond intelligently to the deviations.
The most useful frame is this: HRV is not a performance score. It’s a dialogue with your nervous system. The point isn’t to win the score, it’s to listen to what it’s telling you.
How HRV is measured, and which device to choose
HRV measurement requires high-resolution detection of the time between heartbeats. Two main technologies do this:
ECG-based devices (Polar H10 chest strap, medical-grade equipment): the gold standard. Direct electrical reading of cardiac activity. Highest accuracy.
Optical/PPG-based devices (Apple Watch, Oura Ring, Whoop, Ultrahuman, Garmin): use light to detect blood flow changes through skin. Less precise on individual readings, but very reliable when measuring overnight averages and tracking personal trends over time.
For someone who simply wants to understand their own system, the optical wearables are more than sufficient. The differences in absolute precision matter less than consistency: what you want is a device you’ll wear every night, that captures HRV during sleep when your system is least disturbed.
A practical comparison from my own use:
Apple Watch: measures HRV in random moments throughout the day. Less consistent for trend tracking, but useful if you already wear one.
Ultrahuman Ring (the one I use): continuous overnight readings, daily HRV summary, integration with sleep and recovery metrics. Comfortable and unobtrusive.
Oura Ring: similar approach to Ultrahuman, with a more mature ecosystem and longer track record. Excellent for trend analysis.
Whoop: designed primarily for athletes. Continuous wear, detailed recovery metrics, no display.
Polar H10 chest strap: the most accurate consumer-grade option, but requires wearing a chest strap. Best for people who want clinical-level data on specific moments (training, breathwork, stress tests).
What matters more than the device is the discipline of wearing it consistently for at least four to six weeks before drawing any conclusions. Your baseline emerges from sustained observation, not from a single morning’s reading.
How to actually start using HRV
If this is new to you, here’s a simple sequence:
Weeks 1–4: just observe. Wear the device, record the data, don’t try to change anything. Let your real baseline emerge. Note the average, the range, and how the number feels on each morning.
Weeks 4–8: start correlating. Begin connecting daily HRV with what happened the day before. Late dinner with wine. Bad sleep. A hard workout. A stressful meeting. International travel. You’ll start seeing patterns within the first two weeks.
Weeks 8 onward: use it operationally. When morning HRV is significantly below your baseline, treat the day differently, fewer high-stakes commitments, more recovery work, lighter cognitive load. When HRV is at or above baseline, that’s the day to take on what genuinely demands you.
The trap to avoid: don’t try to optimize HRV. Don’t add ten new biohacks because one morning the number was lower than expected. The goal is to develop a relationship with your own physiology, not to game a score.
Coming next
In the next essay of this series, I’ll get into the part that matters most for those of us building businesses: why HRV is not a wellness metric, it’s a leading indicator of decision quality, and arguably the most important KPI a founder can track.
We’ll look at how the same suppressed HRV that explains a bad sleep also explains a bad pricing call, a soft term sheet, a hire that should have been a no.
Subscribe if you don’t want to miss it.
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Want to read your own physiology with this kind of precision and integrate it into how you operate? Full biological evaluation, expert interpretation and a personal protocol. That’s what we build inside Beyond Human.
In flow,
Rebeca Sanz Barriuso, PharmD, RD, MBA, Neuroscientist
Founder, Beyond Human
References
Amekran, Y., Damoun, N., & El Hangouche, A. J. (2025). A focus on the assessment of the autonomic function using heart rate variability. Global Cardiology Science and Practice.
Liu, S., Cui, Y., & Chen, M. (2025). Heart rate variability: a multidimensional perspective from physiological marker to brain-heart axis disorders prediction. Frontiers in Cardiovascular Medicine, 12: 1630668.
Sundas, A., Contreras, I., Navarro-Otano, J., et al. (2025). Heart rate variability over the decades: a scoping review. PeerJ, 13: e19347.
Wang, B. X., Brennand, E., Le Page, P., & Mitchell, A. R. J. (2026). Heart rate variability in cardiovascular disease diagnosis, prognosis and management. Frontiers in Cardiovascular Medicine.
Olivieri, F., Biscetti, L., Pimpini, L., et al. (2024). Heart rate variability and autonomic nervous system imbalance: Potential biomarkers and detectable hallmarks of aging and inflammaging. Ageing Research Reviews, 101: 102521.
Agorastos, A., Mansueto, A. C., Hager, T., et al. (2023). Heart Rate Variability as a Translational Dynamic Biomarker of Altered Autonomic Function in Health and Psychiatric Disease. Biomedicines, 11(6): 1591.