Partly, and by a large share. In a study of 772 twins and siblings who wore 24-hour heart monitors, genetic factors explained 40 to 48% of the variation in RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD and 35 to 47% of the variation in the SDNN index SDNN index The mean of the SDNN of each 5-minute segment of a 24-hour recording, in milliseconds. It keeps the variability inside each segment and drops the slow drift between them. More about SDNN index . The rest comes from age, fitness, habits and chance, which is why your own trend tells you more than any comparison with other people.
What the twin study found
Kupper and colleagues (2004) recorded a 24-hour ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG in 772 healthy twins and their singleton siblings in the Netherlands and compared how similar identical twins were with how similar fraternal twins and siblings were. The more identical twins resemble each other beyond the others, the larger the genetic share. For the SDNN index, the average of the 5-minute SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN values across the recording, genes accounted for 35 to 47% of the variation depending on the time of day (morning, afternoon, evening and night); for RMSSD, 40 to 48%. The authors concluded that ambulatory HRV is highly heritable, enough to use in genetic studies of cardiovascular risk.
A share of 40 to 48% means that if you lined up a hundred people, roughly half of the spread in their RMSSD would trace to the genes they were born with, and half to everything else.
Which genes, and how much they explain
Finding the genes is harder than measuring their total share. Nolte and colleagues (2017) ran a genome-wide study of three HRV measures in up to 53,174 people of European ancestry and found 17 significant variants in eight regions of the genome, several in genes active in the sinoatrial node Sinus node The heart's natural pacemaker, a small patch of cells that starts every normal heartbeat. Left alone it fires about 100 times a minute; the vagus nerve holds it lower at rest. More about Sinus node , the heart’s pacemaker. Yet a score built from those variants explained only 0.9 to 2.6% of the variation in HRV. Most of the inherited share is spread across many small effects not yet pinned down, as it is for height and blood pressure. The study also found genetic overlap between HRV, heart rate and blood pressure.
The other half
The non-genetic half has one dominant piece: age. Voss and colleagues (2015), in 1,906 healthy adults measured with a 5-minute resting ECG, found a median RMSSD of 36 to 38 ms at 25 to 34 and 17 ms at 65 to 74. Fitness, sleep, alcohol, illness and stress move the number within the range your genes and age set; the what affects HRV guide ranks them.
How much do the measurable factors explain together? In 28,175 people who took a morning reading with a phone app, Altini and Plews (2021) found that age, sex, physical activity level and body mass index together accounted for about 15% of the differences in RMSSD between people. The remaining spread, they argue, is why a higher HRV is not a realistic goal for everyone, and why the number is more useful for tracking change within one person.
What this means for your number
- A low number next to other people’s can be your inherited normal. What a low HRV means separates low-for-others from low-for-you.
- Changes within you are still real. Genes set the level; the week’s sleep, training and drinking move it. The lesson on building your HRV baseline shows how to read those moves.
- Fitness helps, within limits. Does HRV go up when I get fitter? gives the sizes.
When to talk to a doctor
HRV cannot tell you about inherited heart conditions. If heart disease, sudden death or fainting runs in your family, talk to a doctor about screening, whatever your HRV shows.