Heart 
rate 
variability 

Your heart rate variability (HRV) is a powerful metric of your overall health. HRV measures the difference in time between each heartbeat, which fluctuates slightly. High HRV is generally considered better. Low HRV can be caused by medical conditions, stress, or intense fitness training.

The average heart rate variability is

36 ms

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Heart rate variability by age and gender


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?

HRV measures the difference in time between each heartbeat, which fluctuates slightly. So the unit of HRV is millseconds. Your HRV is measured automatically throughout the day by your Apple Watch, including while sleeping. HRV is important primarily because is measures the activity of your autonomic nervous system.

How Apple Watch measures HRV

The Apple Watch measures heart rate variability (HRV) using sensors that track changes in blood volume under the skin with each heartbeat. It calculates HRV by looking at the time between heartbeats (RR intervals).

There are multiple ways to calculate HRV:

  • SDNN (standard deviation of normal-to-normal R-R intervals). SDNN measures cardiac rhythm data throughout the day, and is often used in medical research as a summary over a 24 hour period.
  • 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.

Apple Watch uses SDNN as its measure of HRV. Some third party apps also calculate RMSSD as part of a recovery score (similar to Whoop).

HRV and the autonomic nervous system

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.

What’s a low HRV?

18-33 ms is a low HRV. Depending on age or sex, this would put you in the bottom 10%. An HRV above 76 ms would put you in the 90th percentile.

What’s an average HRV?

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

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 deeper problem.

Gut health

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.

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.

Mental health & 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.

Atrial fib vs atrial flutter on an ECG

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.

What to do if your Apple Watch shows low or high HRV

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.

Supported Apple Watches
Series 1

Series 1

2015

Series 2

Series 2

2016

Series 3

Series 3

2017

Series 4

Series 4

2018

Series 5

Series 5

2019

Series 6

Series 6

2020

SE

SE

2020

Series 7

Series 7

2021

Series 8

Series 8

2022

Ultra

Ultra

2022

Series 9

Series 9

2023

Ultra 2

Ultra 2

2023

Series 10

Series 10

2024

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