About HRV

Your heart rate variability (HRV) is a powerful—if complex—metric of your overall health. We’ll try to give an overview here, but you could really write a whole book on HRV.

What is HRV (heart rate variability)?

A healthy heart is not a metronome: as your heart beats, the timing between each beat shifts slightly. For example, one heart beat may come 900 millseconds after another, and then the next one 950 millseconds after that. That’s healthy and good.

Your HRV measures the variation in time between these heart beats. The unit of HRV is millseconds, and a normal HRV is 18-76ms. Your HRV is measured automatically throughout the day by your Apple Watch, including while sleeping.

While HRV is derived from heart rate, it’s actually a broader window into your overall health. Your autonomic nervous system is the main influence on HRV. Since your ANS is connected to your liver, kidney, stomach, brain, and more, HRV is actually a really powerful metric of many different organ systems in your body. We’ll cover the autonomic nervous system more below, including the how HRV sometimes reflects medical conditions like anxiety, depression, dysautonomia, digestive tract problems, and more.

How Apple Watch calculates HRV

The Apple Watch measures the time of each heart beat using its optical heart rate sensor. Apple Watch calculates HRV using the SDNN formula: the standard deviation between normal heart beat intevals.

SDNN is defined mathematically as:

SDNN = sqrt(Σ(NN_i - mean(NN))² / (N - 1))

Here’s an example. Let’s say your heart is beating around 60bpm. The spacing between heart beats isn’t going to be exactly 1 second—it will vary a little bit beat to beat. Let’s say the first heart beat comes, then a second one 1040 ms later, then a third heart beat 960 ms later, then a final heart beat exactly 1000ms later. Your NN intervals are then 1040ms, 960ms, and 1000ms.

Your average N-N (normal to normal) interval is 1000ms. The standard deviation is 32.6 ms. That standard deviation of 32.6 ms is your heart rate variability (using the SDNN method).

SDNN vs RMSSD

You may have read there are different methods to calculate HRV. The two most common are SDNN and RMSSD:

• SDNN (standard deviation of normal-to-normal R-R intervals). SDNN is a good metric of cardiac rhythm data throughout the day. SDNN is often used in medical research as a summary over a 24 hour period. The Apple Watch’s built-in HRV metric is SDNN.
• RMSSD (root mean square of successive differences between RR intervals). Since RMSSD calculates differences between successive intervals, it captures more short-term, beat to beat variability. This reflects the action of the parasympathetic nervous system (explained in the next section), and is more often used in fitness training. Some third party apps also calculate RMSSD as part of a recovery score (similar to Whoop).

The medical literature has dozens of ways calculate HRV. For example, pNN50 is the percentage of normal-normal intervals that differ by more than 50ms. Frequency domain methods break the NN intervals into waves of different frequencies, and then measure the power in each band. The most typical bands are high frequency (0.15 to 0.4 Hz), low frequency (0.04-0.15 Hz), and very low frequency (VLF) (0.0033 to 0.04 Hz). These aren’t typical used in Apple Watch since they require long measurements of about 5 minutes, which would drain battery.

What’s a low HRV on Apple Watch?

Depending on age and sex, below 18 ms is a low HRV. This is around the bottom 10%. An HRV above 76 ms would put you in the 90th percentile.

What’s an average or good HRV?

Anything between 18 to 76 ms is a normal heart rate variability. Higher is better.

HRV and the autonomic nervous system

HRV is influenced by your autonomic nervous system, which has two branches: the sympathetic nervous system (your “fight or flight” response) and the parasympathetic nervous system (your “rest and digest” response, which is controlled by the vagus nerve). When the sympathetic nervous system dominates, HRV is lower. When the parasympathetic nervous system dominates, HRV is higher.

Causes of low or high heart rate variability

A common cause of temporary low HRV on your Apple Watch is stress or intense fitness training. However, both low and high HRVs can also reflect a medical condition. The most common conditions that affet HRV are dysautonomia, anxiety, depression, nerve damage, abnormal heart rhythms, and some gut health conditions.

Dysautonomia, Long Covid, POTS & low HRV

One of the hallmarks of dysautonomia is imbalance between the sympathetic nervous system (often overactive) and the parasympathetic nervous system (often withdrawn). This leads to low HRV in dysautonomia, including POTS and Long Covid. While HRV itself is not used in diagnosis of POTS, the pattern of heart rate and blood pressure when going from laying to standing is. You can book an appointment with an Empirical Health doctor to get POTS testing and treatment.

Anxiety, depression, & low HRV

Since both branches of the automatic nervous system connect to the brain, low HRV can sometimes be a sign of mental health conditions, such as depression or anxiety. Improving HRV through practices like mindfulness, physical activity, stress management, and medication can support better emotional regulation and mental health.

Nerve damage & low HRV

Nerve damage (for example, from diabetes) can cause low HRV. Nerve damage affects HRV by disrupting the balance and communication between the sympathetic and parasympathetic branches of the autonomic nervous system. When nerves responsible for these regulatory pathways are damaged, the body’s ability to adjust heart rate in response to stressors and environmental changes becomes impaired. This leads to a decrease in HRV.

Abnormal heart rhythms can cause high or low HRVs

Finally, very low or high HRV can also be caused by an electrical problem in the heart (for example, atrial fibrillation, atrial flutter, or supraventricular tachycardia). In these cases, an ECG can give more diagnostic information on exactly what is happening in the heart’s electrical system.

The image above shows two common abnormal heart rhythms, atrial fibrillation and atrial flutter, on an ECG. Atrial fibrillation is highly irregular, leading to high HRV. Atrial flutter is very regular, leading to no or low HRV. Either of these rhythms require attention from a medical professional.

The Apple Watch Series 4 and above include an FDA-cleared ECG sensor that can help distinguish whether low or high HRV is caused by an abnormal heart rhythm.

Digestive tract

Since the vagus nerve is connected with your stomach, intenstines, liver, and so on, low HRV can sometimes be correlated with problems in the digestive tract.

What to do if your HRV is very low (or high)

Because HRV correlates with so much in the body, it’s a powerful metric but doesn’t give specific diagnostic information on exactly what’s wrong. If you have concerns or questions about HRV, it’s best to talk with a doctor about your specific medical situation. You can book an appointment through the Empirical Health app.