Body · lesson 4 of 10
Heart rate variability: a personal number
What HRV measures, why comparing yours with anyone else's is meaningless, and how to use your own baseline.
6 min read · reviewed October 2026
A healthy heart isn't a metronome
Heart rate variability (HRV) is the variation in time between one heartbeat and the next. At 60 bpm the gaps aren't all exactly one second; they might run 0.95, 1.05, 0.98 seconds. Much of that variation comes from the vagus nerve, which speeds your heart slightly as you breathe in and slows it as you breathe out.
Higher resting HRV generally reflects more parasympathetic ('rest and digest') activity. The Stress & Resilience track covers the nervous-system side; this lesson is about measuring it well.
HRV is a within-person metric
HRV depends heavily on age (it falls as you get older), genetics, sex, breathing, posture, time of day, and how the device calculates it. Healthy people of the same age can differ several-fold. Your 35 versus a friend's 80 says almost nothing about who is healthier. Your 35 versus your own usual 55 might say a lot.
RMSSD, overnight averages, and why devices disagree
Most wearables report RMSSD in milliseconds, a measure of beat-to-beat change that tracks vagal activity. Some measure it across the whole night, some during certain sleep stages, some in a short morning reading. Numbers from different devices or different measurement windows aren't comparable.
How it works →
RMSSD is the root mean square of successive differences between normal heartbeats. It reflects fast, vagally driven variation, which is why it works on short recordings. A related measure, SDNN, reflects overall variability over longer recordings. Measuring HRV requires timing each beat precisely, so motion and extra (ectopic) beats add error. That's why still, overnight or seated-morning measurements work best.
What the studies show →
The measurement standards come from a 1996 Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (Circulation, 1996). Shaffer & Ginsberg (Frontiers in Public Health, 2017) reviewed published norms and stressed that recording length, age and sex shape baseline values, and that 24-hour, 5-minute and shorter norms are not interchangeable. In clinical research, low HRV predicts higher mortality; in the Framingham Heart Study, reduced HRV on ambulatory ECG predicted death in older adults (Tsuji et al., Circulation 1994). But those were clinical ECG recordings, and no study shows that raising a wearable HRV number, by itself, extends life.
What pushes your HRV around
Typically lowers overnight HRV and raises heart rate, often visibly even after moderate drinking.
HRV often drops (and resting heart rate rises) around the start of an infection.
A big session can lower the next day's HRV. A return to your usual range suggests recovery.
Short or poor sleep, psychological stress, late heavy meals, heat and dehydration all tend to push it down.
HRV tends to be lower in the second (luteal) half of the cycle.
HRV declines gradually with age. Regular aerobic training tends to raise it over months.
Using HRV well
- Measure the same way every time: the overnight value, or the same morning reading taken lying or sitting still.
- Collect a few weeks of data to learn your normal range before interpreting anything.
- Watch a 7-day rolling average rather than the daily number, the approach used in athlete-monitoring research. Single days are noisy.
- Treat a drop well below your usual range that lasts several days as a prompt to check sleep, alcohol, stress, illness and training load.
- Don't chase the number. Slow breathing can raise a reading in the moment without changing anything about your health.
Your overnight HRV is 38 ms. Your friend's watch says 85 ms. What can you conclude?
- Your friend is much healthier than you
- You're overtrained
- Very little: HRV varies widely between people and devices, so compare against your own baseline
- Your device is broken
Show the answer →
C.Very little: HRV varies widely between people and devices, so compare against your own baseline
HRV is shaped by age, genetics, sex, breathing, and each device's measurement window and algorithm. Comparing between people, or between brands, is close to meaningless. A sustained change against your own baseline is the signal.
The Stress & Resilience track covers what HRV reflects, and the breathing tools that shift you into 'rest and digest'.