PNS Index, SNS Index & Stress Index: The Balance Trio

Three numbers, one balancing act

Open a reading in Autonomic and you will see three balance figures sitting together: the PNS index, the SNS index, and the stress index. They are the app’s compact readout of where your autonomic nervous system is sitting right now: how much of your rhythm is being driven by the rest-and-recover brake, how much by the fight-or-flight accelerator, and how much overall strain the system is under.

They are computed the way the widely used Kubios HRV analysis methods define them, so the numbers you see line up with the clinical literature. The point of showing all three is that no single HRV metric captures balance on its own. Read together, they tell a clearer story than any one number can.

The PNS index: your rest-and-recover side

The PNS index (parasympathetic nervous system index) summarizes how active your rest-and-digest, vagal side is. It is a composite of three things that all rise when the parasympathetic brake is engaged: your mean RR interval (the average gap between beats: longer means a slower, calmer heart), your RMSSD, and SD1 from the Poincaré plot. Those last two are the classic vagal-tone metrics.

The clever part is the scaling. Each input is expressed as a z-score against a healthy reference population, so the PNS index reads like a standardized score: 0 is an average healthy adult, positive means more rest-and-recover activity than average, the good direction, and deeply negative means poor recovery, fatigue or illness.

The SNS index: your fight-or-flight side

The SNS index (sympathetic nervous system index) scores the other branch, the fight-or-flight accelerator. It is a composite of your heart rate, the Baevsky stress index (more on that below), and RMSSD read in reverse. Like the PNS index it is centred so 0 is average, but the direction is flipped: negative means below-average activation, calm, which is good, and a large positive number means stress, exertion or illness.

That reversal trips people up at first. A low SNS index is the desirable one. If your SNS index sits comfortably below zero at rest, your accelerator is idling, exactly where it should be when you are recovered.

The stress index: strain, not balance

The stress index is different in kind from the other two. It is the Baevsky strain index, and it is not centred on zero: it is always a positive number that climbs steeply as the rhythm becomes rigid and concentrated under sympathetic load. Low and stable is the goal.

Where the PNS and SNS indices come from beat-to-beat variation, the stress index comes from the shape of the interval histogram: the most common interval length (Mode), how tall and narrow that peak is (AMo50), and the total spread (MxDMn). When your rhythm is healthy and varied, the histogram is broad and the stress index is small. When the rhythm collapses toward one dominant interval, the peak spikes and the index shoots up. The full mechanics live in the histogram metrics explainer.

Because it responds sharply, the stress index is an early-warning number. A spike often leads your symptoms by a day or two, flagging stress, illness or overreaching before you consciously feel it.

Try a stress-index number against the same bands the app uses:

The see-saw: how the trio moves together

The cleanest way to picture the trio is a balance beam. On the up-side is the parasympathetic PNS index, on the down-side the sympathetic SNS index, and the stress index is the load pressing down on the whole thing.

When you are rested and recovered, the beam sits in its healthy pose: PNS positive, SNS negative, stress index low. When you take on load (a poor night, a hard workout, a flare, an infection) the beam tips: the SNS index and stress index rise together while the PNS index falls. That coordinated flip is the single most useful pattern to learn.

But (and this matters) they are not perfect mirror images. Two caveats keep you honest:

• Both branches can be suppressed at once. After a genuinely hard day or during illness, the whole system can go quiet: PNS low and SNS not as high as you would expect, with the stress index still elevated. Do not assume a low PNS always means a high SNS.
• They answer slightly different questions. PNS and SNS describe balance; the stress index describes strain. You can be balanced but strained, or unbalanced with modest strain. Reading all three is what resolves the ambiguity.

Reading the trio in recovery

In POTS, long COVID and post-viral dysautonomia the resting pattern is often skewed toward the sympathetic side: a suppressed PNS index, an elevated SNS index, and a higher stress index than a healthy baseline. That is the “stuck on” bias these conditions are known for, and it is exactly what the trio is good at showing.

The encouraging part is that the pattern shifts as you recover. Over months of pacing, many people watch their resting PNS index drift upward, their SNS index settle lower, and their stress-index spikes become smaller and rarer, often before they feel the change. Read these against your resting heart rate and the complete HRV picture, and follow the multi-week trend rather than any single morning. We map that arc in recovery from post-viral dysautonomia.

The bottom line

The PNS index, SNS index and stress index are three views of one balancing act: the PNS index scores your rest-and-recover side (0 is average, positive is good), the SNS index scores your fight-or-flight side (0 is average, negative is good), and the Baevsky stress index measures how rigid your rhythm has become under load (always positive, low is good). Recovered, they sit in a stable pattern; under strain, the pattern flips together. They are not perfect mirrors, and both can sag at once after a hard day, so read all three, follow the trend, and let the consensus between them, not any single number, tell you where your nervous system is heading.