Your watch gave you a stress score. Here is what it is actually counting

Written byAnastasia DeripaskaPublished4 October 2026 Reading6 min

A wrist wearing a plain fitness watch resting on a cream linen duvet in soft morning light

Your watch now hands you a stress score before you have finished your coffee. Over the past twelve months, US searches for "heart rate variability" rose by roughly three quarters and searches for "stress score" nearly tripled compared with the year before, according to Google Trends. The number arrives with a color, a verdict and sometimes a nudge to take it easy. Here is what is behind it.

Heart rate variability: a healthy heart is not a metronome

Heart rate variability, or HRV, is the variation in time between one heartbeat and the next, measured in milliseconds. If your heart beats 60 times a minute, the gaps are not all exactly one second. Some are a little longer, some a little shorter, and that wobble turns out to be informative. A widely cited review in Frontiers in Public Health explains that it reflects the constant negotiation between the two branches of the autonomic nervous system, the part that runs your body on autopilot. The sympathetic side prepares you to act. The parasympathetic side, carried largely by the vagus nerve, slows things down. Breathing alone shows it in action: the heart speeds up slightly as you inhale and eases off as you exhale.

In general, higher resting HRV is associated with a body that can shift gears easily, and lower HRV with one that is working harder or under strain. But the same review stresses how much context matters: age, sex, the length of the recording and the time of day all change what "normal" looks like.

Our earlier piece on the vagus nerve covers that part of the story.

A stress score is a recipe, not a measurement

HRV is a measurement. A stress score is an interpretation. Wearable brands combine HRV with other signals, typically resting heart rate and sometimes movement, temperature or sleep, and run them through their own formulas to produce a single number. Those formulas differ by brand and are not publicly standardized, so a 62 on one device and a 62 on another are not the same thing.

A bedside table at dusk with a glass of water, an open book and a fitness watch on its charger

How accurate is the number on your wrist?

It depends on the device and on when it measures. A 2025 study in Physiological Reports had 13 healthy adults wear an ECG reference alongside several consumer wearables across 536 nights of sleep. Oura's third and fourth generation rings tracked overnight HRV most closely in that study, WHOOP showed moderate agreement, and the Garmin and Polar watches tested agreed poorly. The Apple Watch was not part of that study, and its results apply to those specific models and a small group of healthy adults. The practical lesson: compare a device with itself over time. The study tested overnight readings only, taken while people were still, so those are the readings it can vouch for.

What actually moves it

Many everyday factors shift HRV, which is one reason single readings mislead. Alcohol shows up clearly in observational data. In a study of 4,098 Finnish employees, each person compared with themselves on nights with and without a drink, alcohol lowered HRV and raised heart rate during the first three hours of sleep.

The effect grew with the dose: the rest-and-recover measure fell by about 2 milliseconds after a low intake and by almost 13 after a high one.

Breathing moves it in the other direction, at least in the moment. Slowing down to around six breaths a minute brings heart rhythm and breathing into step.

That produces large, regular swings in heart rate, a phenomenon described in a review of HRV biofeedback in Frontiers in Psychology. Training people to breathe this way with real-time feedback has been studied for years: a meta-analysis in Psychological Medicine pooled 24 mostly small studies with 484 participants and found a large reduction in self-reported stress, while noting that better-controlled studies are still needed.

What to do with your number

As a rule of thumb, look at the trend, not the day: weekly averages of overnight readings say more than any single morning.
Compare a device only with itself. Numbers from different brands do not line up.
Notice what seems to move it for you, such as a late drink or a short night.
If you want to try moving it, a few minutes of slow breathing at around six breaths a minute is the simplest experiment.
If you are choosing a device, the two ring generations tested tracked overnight HRV most closely in the 2025 comparison, but wearing the same one every night matters more than the brand.

A woman sitting by a bright window with her eyes closed and a hand on her chest, breathing slowly

Where the number misleads

HRV is personal. Comparing your number with a friend's says very little, because baselines differ widely between people. As a rule of thumb, a single low morning is mostly noise; trends over weeks carry more meaning. A stress score is an estimate built on someone else's formula, not a diagnosis, and a low reading is not a reason for alarm. If a number worries you, or you feel unwell, that is a conversation for a doctor, not an app.

There is also a risk in watching too closely. Sleep researchers coined the term orthosomnia for people whose pursuit of perfect sleep data starts to disturb the sleep itself. The same trap is open for stress scores: chasing a better number can become one more source of stress.

Beyond the score

HRV also shows up in research on felt sound. Our comparison of sound baths and vibroacoustic therapy walks through some of those studies, including what they did not find.

Used well, HRV prompts one simple question: what helps me come back down? For some people it is a walk, for others slow breathing, music or an earlier night.

This article is for general information only and is not medical advice. Talk to a qualified health professional about any health condition.