The Nervous System Has a Vote

On the body's report: it tells you how loud it is in there. It does not tell you what is playing.

The number that would not explain itself

In the 1990s I tested athletes in a lab. What follows is a constructed composite, built on that true fact. The protocol is the same each time: the load steps up on a schedule, the heart rate climbs on the screen, and the numbers are supposed to tell you what the training did.

Most days they do. Some days they don't. A person comes in on the same block of work, same protocol, same hour of the morning, and the result sits somewhere the training cannot explain. Nothing in the training log accounts for it. What has changed is the person's emotional state. Sometimes the state is a hard one. Sometimes it is a good one.

In the composite, the number can move the same way for the good state and the hard one. The heart says something is going on. It does not say what.

The true part is what the lab taught me: the nervous system has a vote in every test, and its ballot is not labeled. Later, working in exercise as medicine, I found the same thing in people recovering from illness and injury. Their emotional interior mattered to recovery as much as the exercises and the medicine did.

Now we wear the lab on our wrists. A ring scores your readiness. A strap scores your recovery. A watch scores your stress. This article is about what those numbers can honestly say, and the one thing they cannot: which feeling you are having, and what it means.

What the body actually reports

Heart rate variability, or HRV, is the small change in time between one heartbeat and the next. A healthy heart at rest is not a metronome. It speeds a little as you breathe in and slows as you breathe out, and much of that fast wobble is carried by the vagus nerve, which slows the heart. Shaffer and Ginsberg's 2017 overview treats RMSSD, a measure of beat-to-beat change, as the main index of that vagal influence, and it is the number most wearables report. The same review calls the popular "LF/HF ratio," often sold as stress versus rest, controversial, and warns that higher HRV is not always better: some rhythm disorders inflate it. The same overview says the two branches of the autonomic nervous system interact in complicated ways, so no single number is a teeter-totter between bad stress and good calm.

Laborde, Mosley and Thayer's 2017 guide for researchers links cardiac vagal tone to self-regulation, but rests that link on theoretical models, not settled fact. Their list of what moves HRV with no change in your inner life is long: age, sex, breathing rate, sleep, caffeine, alcohol in the past day, a recent meal, yesterday's hard session, even a full bladder. A low morning number may be a hard day coming. It may be last night's wine.

Load and recovery: useful, modestly. The best-supported use of HRV is guiding endurance training. Daniel Plews, working with elite athletes, recommended a rolling average of at least three mornings a week, judged against the athlete's own smallest worthwhile change, because single days are noisy; his samples were small, from two to ten athletes. A 2020 meta-analysis by Granero-Gallegos and colleagues pooled six randomized trials, 195 runners, cyclists and skiers, mostly eight weeks. VO2max rose in both groups, more when daily training followed HRV (effect size about 0.40) than with a fixed plan (about 0.22). Six small trials, most with high or unclear risk of bias, so I read it as a modest edge, larger for amateurs than elites.

Your line, not the population's. HRV differs a great deal from one healthy person to the next, with age, sex, fitness and genes. Published norms describe groups. They cannot tell you whether your number is good for you. A reading that looks low on a chart of averages may be an ordinary morning for you, and a reading that looks fine may be a real drop from your own usual. That is why the useful comparison is you against your own recent weeks. The trend is the signal. The single number, set against strangers, mostly is not.

Overtraining: no number catches it. The 2013 joint consensus of the European College of Sport Science and the American College of Sports Medicine says no diagnostic tool rules in overtraining syndrome; it is diagnosed by excluding other causes, often in hindsight. HRV findings were inconsistent. Mood questionnaires tracked training load more faithfully, though people can fake them in either direction. In the hardest case, one of the best signals is a person saying how they feel.

The body does not name the feeling. In 2018 Erika Siegel, Lisa Feldman Barrett and colleagues pooled more than 200 studies and over 8,400 adults, looking for autonomic fingerprints of anger, disgust, fear, happiness and sadness. They found no one-to-one mapping: responses varied widely within each emotion and overlapped between them. These were short lab inductions in healthy adults, and the authors say their test of the alternative view is limited. It still fits the lab. The body reports arousal and load. The naming happens in context, by a person.

So HRV, heart rate and the scores built from them cannot, on their own, identify an emotion. They can tell you the system is working hard or settling. They cannot tell you whether that is fear, eagerness, grief or a hill.

They also cannot tell you whether you are ready. Arousal is how revved the system is. Readiness is whether you can bring what this task needs. A high heart rate before a race can be a body getting ready to work, and a calm morning number can sit on top of a mind that is nowhere near the start line. The first is physiology. The second also depends on sleep, skill, attention and what the event means to you.

Imagine two rowers on the start line, side by side, both with a heart rate of 150 and a low HRV reading that morning. One is thrilled. She has waited all season for this race. The other is dreading it, sure he will let the boat down. Their wrists would report the same morning. Their insides would not. That is a constructed scene, and every coach has seen it.

Sensing the inside

There is a second layer. The brain does not only receive the body's signals; it interprets them. Interoception is the name for sensing, interpreting and integrating signals from inside the body. A 2018 consensus review by Khalsa and colleagues, from a single summit, treats it as several things at once: how accurately you detect a signal, how much you tend to attend to it, and how well your confidence matches your accuracy. The measures only partly overlap, and the field has no agreed taxonomy. So "listen to your body" is not one skill. It is a practice, and the two-column week below is one small way to try it.

Breath is the lever you hold. HRV biofeedback teaches people to breathe slowly, at the pace where the heart's rhythm swings widest. Goessl, Curtiss and Hofmann pooled 24 studies, 484 people, in 2017 and found a large drop in self-reported stress and anxiety versus control (Hedges' g 0.83), while asking for better-controlled trials. Lehrer and colleagues in 2020 pooled 58 randomized studies and found a small-to-moderate effect, smaller against active controls, with the largest effects on anxiety, depression, anger and performance, each from only a few studies. Promising, not proven, and real.

What the body reports, and what it cannot tell you

Read it left to right: the signals carry real information about how much is happening, and the last step, which feeling and why, always comes from you.

Reading the dashboard

Every number below is worth having. None of them is a feeling. The dashboard is useful because it tells us something about the machine. It becomes dangerous when we mistake the dashboard for the whole journey. This is my reading of how to hold each one, not a clinical guide.

The accuracy question comes first. A 2025 validation study compared five devices with an ECG over 536 nights. Resting heart rate was off by about 2 to 3 percent or less (Garmin's was excluded); nighttime HRV error ran from about 6 percent (Oura Gen 4) to about 16 percent (Polar Grit X Pro), with WHOOP and Garmin between. That is 13 healthy adults, sleeping, so daytime readings are likely rougher. The devices also differed from one another, and a readiness score on top is a company's blend of them. A precise-looking number can hide a lot of uncertainty.

The coxswain reads the face

In a racing eight the coxswain sits in the stern, facing the crew, close enough to read a face. No monitor. Just a face. As Daniel James Brown tells it in The Boys in the Boat, Bobby Moch watched a sick stroke that way in Berlin in 1936. What a face reports, a wrist cannot: whether a tired or frightened rower still has something left.

A wearable on a sick rower would likely say: do not race. It would often be right about the body. It could not know what the boat means to the person in it. I hold that as a story about the vote, not a case for ignoring a sick body. Most days the body's vote should win.

The old traditions noticed that breath is the one part of this machinery you can steer. Patanjali's Yoga Sutras treat pranayama as the easing of inhalation and exhalation, and say it makes the mind fit for concentration.

I don't think the HRV studies confirm the Sutras. They rhyme. Breath is a door between the body's weather and the attention you bring to it, and the door does not tell you what to feel.

Where SportsFlow fits

SportsFlow reads the devices this article has been weighing: WHOOP, Oura, Garmin, Polar and Apple Health, with Concept2 erg data on its own line. It reads them against your own rolling 28-day baseline, not a population norm, close to the advice from Plews above. With fewer than ten days of history it shows your self-report only, because a baseline that young is mostly noise.

The design calls self-report and body "two witnesses." You answer first, before you see any wearable number, so the number cannot write your answer for you. When the two disagree, the gap is information; it is never treated as you being wrong. Some scores, self-kindness and gratitude among them, carry no body weighting at all, because no wearable can measure them honestly. And nothing in it claims to read which emotion you feel from your body. Siegel's review is the reason.

There is also a return-to-baseline view: how quickly arousal settles after an event you mark. Your coach never sees it. These capabilities are in development and are not clinically validated. The experiment below is the same guess-then-check, done by hand.

Still not enlightened, Orion