Yes, it is normal for the time between heartbeats to change. A healthy heart is not a metronome: even when you sit still, each gap between two beats is a little longer or shorter than the one before.
In this lesson you will see why your heart rate is only an average, how your breathing pushes the beats closer together and further apart, and how to tell this normal change from symptoms that need a doctor.
Your heart rate is an average
Your heart has its own natural pacemaker, a small group of cells in the upper right chamber. With each beat, an electrical signal starts there and spreads down through the heart, which makes it squeeze. At rest, a heart rate of 60 to 100 beats per minute (bpm) is normal for adults.
A number like 60 bpm sounds as if the heart beats once every second, on the dot. It doesn’t. 60 bpm only tells you how many beats fit into a minute. The beats themselves are spaced unevenly.
Here are five gaps between beats from a person resting at about 60 bpm:
| Beat | Gap in seconds | Gap in milliseconds |
|---|---|---|
| 1 to 2 | 0.95 | 950 |
| 2 to 3 | 1.04 | 1,040 |
| 3 to 4 | 0.98 | 980 |
| 4 to 5 | 1.06 | 1,060 |
| 5 to 6 | 0.97 | 970 |
The five gaps add up to 5.00 seconds, so the average is exactly 1 second, or 60 bpm. Yet no two gaps are the same. The shortest and longest differ by 0.11 seconds.
Researchers who study the heart describe the normal resting rhythm as “highly irregular” when you look at it beat by beat. Your watch or app smooths this out when it shows you a single heart rate.
Zoom in: the time between beats
To see the gaps, you need to measure time in small units. HRV uses milliseconds (ms). One millisecond is a thousandth of a second, so 1.04 seconds is 1,040 ms.
On 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 , the heart’s electrical signal makes a tall, sharp spike with every beat. This spike is called the R peak. The time from one R peak to the next is the RR interval RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval . Every HRV number is built from these intervals.
An electrocardiogram trace with four tall spikes, the R peaks. The gaps between them are labelled 0.98, 1.06, 0.95 seconds, so no two gaps are the same.
Look at the gaps under the trace. They read 0.98, 1.06 and 0.95 seconds. The heart rate barely changes over these few beats, but the spacing of the beats does.
Heart rate and beat timing answer two different questions:
- Heart rate asks: how many beats happen in a minute?
- Beat timing asks: how long is each gap, and how much does it change from one gap to the next?
Heart rate variability (HRV) is about the second question. In the next lesson you will see how a string of RR intervals becomes one picture, and then one number. If you want to compare the two ideas now, read HRV vs resting heart rate.
Breathe in, breathe out
The biggest reason the gaps change at rest is your breathing. When you breathe in, your heart speeds up a little, so the gaps get shorter. When you breathe out, it slows down, so the gaps get longer. Scientists call this respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia . Despite the name, it is normal.
You can feel it yourself. Breathe slowly and steadily, and keep a finger on your pulse. With slow, deep breaths the effect is easier to notice than with quick, shallow ones. The practice task at the end of this lesson walks you through it.
The pattern repeats with every breath. So if you breathe about 12 times a minute, your heart rate rises and falls about 12 times a minute too. Your nervous system does this on autopilot, and lesson 3 explains which nerves are involved.
Go deeper Why does the heart follow the breath?
Nobody knows the full answer yet. One leading idea is that the pattern helps the heart and lungs work as a team. Grossman and Taylor (2007) suggest it helps line up breathing and blood flow as the body’s energy needs change. Their review also warns that the size of this breathing effect depends on how fast and deep you breathe, so it is not a pure measure of anything on its own.
Why a little variation is a good sign
A heart that can speed up and slow down from one beat to the next is a heart that is responding. Your body is always adjusting: to breathing, to standing up, to a noise in the next room. Small, constant changes in beat timing are one sign that these adjustments are working.
In research, lower HRV has been linked with a higher risk of future health problems. More variation is not always better, though. Some kinds of extra variation come from messy data or from rhythm problems, not from a healthy heart. Lesson 4 covers this in detail.
For now, remember that in a healthy person at rest, the gaps between beats are supposed to change.
When uneven beats are something else
Normal beat-to-beat change is small and smooth. You usually cannot feel it at all unless you pay close attention while breathing slowly.
Some sensations are different, and HRV does not measure them:
- a feeling that your heart is pounding or racing
- a feeling that your heart skipped a beat or added an extra one
- a pulse that seems fast and chaotic, with no steady pattern
These feelings are called palpitations. Most of the time they are not serious. But some come from an abnormal heart rhythm, and an HRV app cannot tell the difference.
Get emergency help if palpitations come with fainting or nearly fainting, chest pain or discomfort, trouble breathing, unusual sweating, or dizziness.
See a doctor soon if you often notice extra beats, if the feeling is new or different, if you have heart disease, or if your resting pulse Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate stays above 100 bpm without exercise, fever or anxiety.
An HRV number cannot diagnose anything. It describes beat timing. A doctor can check your heart rhythm properly with an ECG.
What’s next
In the next lesson, What HRV is, in one picture, you will put those gaps on a chart and see how an app turns them into a number. For the full background, see what HRV is.