Usually it doesn’t: a lower resting heart rate 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 normally comes with a higher HRV. When both fall together in a fit person, the likeliest reason is parasympathetic saturation Parasympathetic saturation When the vagal brake is so strong, usually in very fit people with very slow resting heart rates, that the beat-to-beat swing shrinks. HRV then falls while vagal activity is rising. More about Parasympathetic saturation , where the vagal brake is pressed so hard, below about 60 beats a minute, that the beat-to-beat swing shrinks. Fatigue can look similar, so read both numbers together.
The normal pattern: slower heart, higher HRV
Two things tie HRV to heart rate, as Jerzy Sacha (2014) explains. The first is physiology: 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 slows the heart and adds variation between beats at the same time. The second is maths. The gap between beats in milliseconds is 60,000 divided by heart rate, and that curve is steep at low rates:
| Heart rate | Average gap | One beat a minute is worth |
|---|---|---|
| 50 bpm | 1,200 ms | about 24 ms |
| 60 bpm | 1,000 ms | about 17 ms |
| 100 bpm | 600 ms | about 6 ms |
So the same small nudge from your breathing shows up as a bigger RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD when your heart is slow. Slide the heart rate below to see it. With saturation off, RMSSD falls steadily as heart rate rises.
The same nudge, a slower heart
A teaching model, not a measurement
- Mean gap
- 1000 ms
- One beat a minute
- 17 ms
- RMSSD
- 42 ms
- lnRMSSD
- 3.73
- lnRMSSD:RR ×1000
- 3.7
At 60 bpm the average gap is 1000 ms and one beat a minute is worth 17 ms. The same 6 beats a minute swing gives an RMSSD of 42 ms; lnRMSSD is 3.73 and the lnRMSSD:RR ratio 3.7.
The slider works when JavaScript is on.
A chart of RMSSD against heart rate from 40 to 100 beats a minute. Without saturation the curve falls steadily: about 60 ms at 50, 30 ms at 70 and 15 ms at 100. With saturation on, a dashed second curve bends down below 60 beats a minute. A slider sets the heart rate, a checkbox turns saturation on, and readouts give the mean gap in milliseconds, the milliseconds per beat a minute at that rate, RMSSD, lnRMSSD and the lnRMSSD:RR ratio.
When both fall together: saturation
At the slow end the pattern can turn around. Buchheit (2014) explains that in some trained people, once the gaps between beats are longer than about 1000 ms, the vagal brake is so strong and steady that there is little room left for it to ease off with each breath. The heart is slow and very even, and RMSSD drops. The drop comes from more vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone , not less, because HRV measures the size of the swings in vagal outflow rather than its level.
Plews and colleagues (2013) describe this in elite endurance athletes: HRV falling while resting heart rate keeps dropping. Quigley and colleagues (2024) note that the breathing swing is “nearly abolished” at very high vagal activity and advise researchers to check for this ceiling. Tick “saturation” in the model above and slide the heart rate under 60 to see the curve bend down.
Saturation or fatigue?
A falling HRV can also be a warning sign of too much training. These clues help tell them apart:
| Clue | Points to saturation | Points to fatigue |
|---|---|---|
| Resting heart rate | Falling, or very low and stable | Often higher than usual |
| How you feel and train | Well, performance holding | Tired, sore, sessions going badly |
| lnRMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD :RR ratio | Dropping while the gaps lengthen | Very large rises may point to poor adaptation |
Buchheit recommends tracking lnRMSSD divided by the mean RR Mean RR The average time between heartbeats in a recording, in milliseconds. Heart rate is the same number upside down: 60,000 divided by the mean RR. More about Mean RR interval. In saturation the gaps get longer while the ratio drops; he adds that extreme increases in the ratio might reflect poor adaptation to training. The model above shows the ratio as “lnRMSSD:RR ×1000”.
Go deeper Should HRV be corrected for heart rate?
Some researchers divide HRV by the mean gap between beats, or by its square, to remove the part that comes from heart rate itself. De Geus and colleagues (2019) reviewed these methods. They favoured the simplest, expressing HRV as a percentage of the mean gap (a coefficient of variation), noted that it weakens the link with heart period without removing it, and advised always reporting results with and without any correction. They warned against “regressing out” heart rate in statistics. The question of how much of the link is a direct effect of heart period is still open.
What to do
- Read HRV and resting heart rate together, as 7-day averages, never one day alone. See HRV vs resting heart rate.
- If you are not a highly trained endurance athlete, saturation is unlikely. A drop in HRV with a falling heart rate then deserves the same checks as any drop: sleep, alcohol, illness, training load.
- Talk to a doctor if a very slow heart rate comes with dizziness, fainting, breathlessness or chest pain. An HRV number cannot check your heart rhythm.
The lesson why higher HRV is not always better covers the other ways HRV and heart rate interact.