HRV: what heart rate variability tells you

Same heart rate, different intervals

Your heart does not beat like a metronome. The autonomic nervous system adjusts every beat: the parasympathetic branch, in charge of rest and recovery, makes the intervals vary more. The sympathetic branch, in charge of tension and effort, makes the rhythm faster and more even^{1, 2}.

A higher HRV than usual therefore usually points to a relaxed, recovered body, and a lower one to strain: a hard workout, a short night, alcohol or an infection starting.

Normal is what is normal for you

There is no general normal value for HRV. It depends strongly on age and sex, and values vary widely among healthy adults^{2, 3}. 35 ms can be good for one person and low for another.

What matters is the comparison with yourself: how does today’s value compare with your last few weeks? This is also how sports scientists follow athletes’ HRV^{4, 5}. Single nights vary by a few per cent without meaning anything. The trend over several days says more.

What affects your HRV

• ↓lowers: alcohol in the evening⁶
• ↓lowers: hard training, in the days after^{4, 5}
• ↓lowers: an infection⁷
• ↓lowers: the second half of the cycle⁸
• ↓lowers: older age³
• ↑raises: regular endurance training, over weeks⁹

Alcohol has a clearer effect than many people think: in an analysis of more than 4,000 employees, recovery during the first hours of sleep was measurably worse even after a little alcohol, and the more they drank, the stronger the effect⁶.

In women, HRV often drops a little in the second half of the cycle⁸. That is not a sign of poor recovery but a normal rhythm.

How your watch measures HRV

Watches and rings measure the pulse optically at the wrist or finger and compute the intervals between beats. Several metrics can be derived from them. The two most common are RMSSD, which mainly reflects the fast, parasympathetic variations, and SDNN, which also includes slower influences².

What Salvor does with it

Salvor learns your own range over the first 14 nights and compares every night with it. Only the deviation from yourself counts, never an absolute value. Salvor keeps values of different metric types apart, so switching devices does not look like a crash.

HRV makes up 40 % of the body signals, next to resting heart rate, breathing rate and temperature. Together with sleep and training balance, the body signals are the basis of your daily form.

When to see a doctor

A low HRV on its own is not a diagnosis. After a short night, alcohol or hard training it is quite normal. If you have symptoms such as a racing heart, dizziness, shortness of breath or chest pain, talk to a doctor, whatever a watch shows. In an emergency, call 112.