HRV, short for heart rate variability, is how much the time between heartbeats changes from one beat to the next. Put each gap between beats on a chart, one after another, and HRV is how much that line wiggles.
In lesson 1 you saw that your heart does not tick like a clock. This lesson turns that idea into a single picture. Once you can read it, most HRV numbers start to make sense.
The picture: gaps between beats, one after another
Start with a heart recording. An 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 records the heart’s electrical activity, and each beat shows up as a tall spike. A watch, ring or chest strap does the same job in a simpler way: it notes the moment of each beat.
Now measure the gap from each beat to the next, in milliseconds (ms). You get a list like this: 1,004, 1,034, 1,020, 987, 969 and so on. Then draw it:
- Along the bottom, put the beat number: first gap, second gap, third gap.
- Up the side, put the length of each gap in milliseconds.
- Join the points with a line.
That chart is called a tachogram Tachogram A line chart of heartbeat intervals in the order they happened, one point per beat. It shows HRV as a wavy line instead of a single number. More about Tachogram . Every HRV number is a summary of a chart like this.
When the line goes up, the gaps are getting longer and the heart is slowing down. When it goes down, the gaps are getting shorter and the heart is speeding up. At rest, the line often rises and falls in a gentle wave that follows your breathing.
Now look at the size of the wiggle. A line that swings up and down a lot means the gaps change a lot, so variability is high. A line that is almost flat means each gap is nearly the same as the one before, so variability is low.
Try it: make the line wiggle
The chart below shows sixteen beats. The marks along the top show when each beat happens, and the line below shows the length of each gap. Drag the slider to change how much the gaps vary.
- RMSSD
- 30 ms
- SDNN
- 24 ms
- Heart rate
- 60 bpm
An interactive chart of sixteen heartbeats. A slider controls how much the gaps between beats vary. The RMSSD, SDNN and heart rate values update as you move it.
Try these three things:
- Move the slider all the way to the left. Every gap is exactly 1,000 ms. The line is flat and both HRV numbers drop to 0 ms.
- Move it to the middle. The line starts to wave, and the HRV numbers climb.
- Move it all the way to the right. The line swings hard, and the numbers are at their highest.
Now look at the heart rate box while you do this. It stays at about 60 bpm the whole time. The heart is beating just as fast. Only the spacing of the beats changes.
Same heart rate, different HRV
Two people can both have a heart rate of 60 bpm while one has much higher HRV, because HRV measures how much the gaps vary, not how fast the heart beats.
Here is a made-up example to show the idea. Both people have five gaps that average 1,000 ms, which is 60 bpm:
| Gap | Person A (ms) | Person B (ms) |
|---|---|---|
| 1 | 995 | 940 |
| 2 | 1,005 | 1,060 |
| 3 | 1,000 | 970 |
| 4 | 990 | 1,050 |
| 5 | 1,010 | 980 |
| Average | 1,000 | 1,000 |
Person A’s gaps change by 5 to 20 ms from one beat to the next. Person B’s change by 70 to 120 ms. On a chart, A would be a nearly flat line and B would be a big zigzag.
This is why HRV and heart rate are worth tracking as two separate numbers. If you want the longer comparison, read HRV vs resting heart rate.
From picture to number
An app cannot show you a chart every morning and ask you to judge the wiggle. It needs one number. So it does roughly this:
- Look at each gap and the one right after it.
- Note how different they are.
- Average those differences over the whole recording.
The result is a number in milliseconds. Big jumps between neighbouring gaps give a big number. Small jumps give a small one. This measure is called RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD . It is the main measure researchers use for quick, beat-to-beat changes. The explorer above shows it, along with a second measure called 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 , which looks at how spread out all the gaps are.
You don’t need the maths yet. Lesson 5 walks through the calculation with real numbers, step by step.
Apps use different measures and different scales, so a number from one app is not always comparable with a number from another. Some apps show RMSSD in ms. Others show a score. That is why the practice task asks you to write down the unit.
What the wiggle is telling you
Your nervous system makes most of the wiggle. At rest, much of the quick beat-to-beat change comes from the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve , which can slow the heart in under a second. That is why RMSSD is often used as a sign of how active the body’s “ rest and digest Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system ” side is.
When you are relaxed, the variation between beats tends to be larger. When the body is in more of a fight-or-flight Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system mode, it tends to be smaller. Lesson 3 explains how this works. Lesson 4 explains why a higher number is not always a better one.
What the picture can’t show
A tachogram is a snapshot. It shows how much your gaps changed during one recording, and that’s all. It does not show:
- Why your HRV changed. Sleep, a late meal, coffee, alcohol and hard training the day before can all shift it. That is why researchers ask people to avoid them before a test.
- Whether your number is good for someone of your age. People’s normal ranges differ a lot. See what a good HRV is.
- What happens next. One reading is one moment. Patterns over days and weeks tell you far more.
For the full background, see what HRV is. In the next lesson, the nervous system behind HRV, you will meet the nerves that make the line wiggle.