Your heart does not beat like a metronome. The gap between one beat and the next changes slightly, all the time. Heart rate variability, HRV, is the size of that variation. A resting heart at 60 beats a minute does not beat exactly once a second. One gap might be 0.98 seconds, the next 1.04. HRV measures how much those gaps move around.
Why the gaps change
Two branches of your nervous system share the wheel. The sympathetic branch speeds things up: stress, effort, a deadline, a fright. The parasympathetic branch slows things down: rest, digestion, recovery. When the parasympathetic branch is in charge, it nudges the timing of each beat with your breathing and other rhythms, and the gaps vary more. When the sympathetic branch dominates, the heart runs more like a metronome and the gaps vary less.
So, as a rule of thumb: more variation means a body that is resting and recovering, less variation means a body under load. That is why HRV became the go to signal for recovery in wearables.
How it is measured
The common short term measure is rMSSD, the root mean square of successive differences. Take every pair of adjacent beats, work out how much the gap changed, square it, average it, take the square root. The result is in milliseconds. It is the measure the 1996 Task Force guidelines recommend for short recordings and the one most wearables report, including Sixteen.
Apple Watch reports SDNN, the standard deviation of all the gaps in a short window. It is related but not identical, which is one reason numbers differ between devices. Never compare an HRV from one brand with an HRV from another.
Where the number comes from matters as much as the maths. Overnight readings, pooled across the whole night, are far steadier than a 60 second test in the morning. Sixteen scores overnight HRV for that reason.
What is normal
Honestly, a wide range. Shaffer and Ginsberg’s 2017 review collects norms from many studies and the spread is large: healthy adults with rMSSD anywhere from the low 20s to above 100 ms. HRV falls with age, is a little higher in women at rest, and is higher in people with good aerobic fitness. Genetics play a part. None of that makes a low number a problem.
The only comparison that means anything is you against you. A seven day average with a normal band around it tells you whether tonight was typical. A single reading tells you almost nothing.
What moves it day to day
Down, usually: alcohol the evening before, a late heavy meal, a hard training session, poor or short sleep, illness coming on, heat, dehydration, psychological stress, screens late at night.
Up, usually: a full night of sleep at a consistent time, easy aerobic exercise, hydration, a calm evening, breathing practice.
Notice that most of these are things you did yesterday. HRV is a report on your last 24 hours more than on your fitness.
What HRV is not
It is not a diagnostic test. A low HRV does not mean you are ill and a high one does not mean you are well. It cannot tell you why it moved. Wearable HRV should never be used to make decisions about medication or a medical condition. If your resting heart rate and HRV shift for weeks without an obvious cause, see a doctor.
How Sixteen uses it
Sixteen measures overnight rMSSD, builds a rolling baseline with a smallest worthwhile change, and shows each day as very high, high, normal or low against that band. HRV is the heaviest input to the recovery score, which sets the strain target for your day. When HRV is low, Sixteen suggests an easier day and holds your evening app lock a little earlier. The number becomes a decision, not a widget.
Sources
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation, 1996.
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017.
- Plews DJ et al. Training adaptation and heart rate variability in elite endurance athletes. Sports Medicine, 2013.
