Everyone should understand their own heart rate variability (HRV), because it is one of the few signals that shows, night by night, how your body responds to sleep, alcohol, training, stress and illness. You do not need to be an athlete or a scientist to use it, but you do need to know what the number is, what moves it, and what it can never tell you.
What HRV is, in one paragraph
Your heart does not beat like a clock, so the gap between two beats might be 0.95 seconds, then 1.02 seconds, then 0.98 seconds. HRV measures how much those gaps change from one beat to the next. Much of that change comes from your nervous system adjusting your heart rate from moment to moment, which is why HRV gives a rough view of how your body is coping. A healthy heart is not a metronome, and the amount of variation you see depends on your age, your sex and how long the recording lasts (Shaffer and Ginsberg, 2017). For the full picture, read what HRV is.
What your HRV can show you
The useful part of HRV is not the number on any one day but the way it moves away from your own normal range and then comes back. One large study looked at 9 million morning readings from 28,175 people who measured themselves with a smartphone app when they woke up, over up to five years (Altini and Plews, 2021). It found clear changes after common events such as hard training, heavy drinking and illness, and the examples below show what those changes look like in everyday life.
After a few drinks
Say you have three glasses of wine at a friend’s dinner on a Saturday evening, and then go to bed a little later than usual. That night your heart rate tends to stay higher while you sleep, and your overnight HRV tends to drop below your usual level.
In a study of 4,098 Finnish employees, the researchers compared each person’s nights after drinking with their own nights without alcohol (Pietilä and colleagues, 2018). A measure of recovery based on HRV during the first three hours of sleep fell by about 9 percentage points after a low dose, 24 after a moderate dose and 39 after a high dose. Regular physical activity did not protect people from the effect, which was similar for women and men. In the large app study, a night of heavy drinking went with HRV about 12% lower and heart rate about 6% higher the next morning (Altini and Plews, 2021). Read more in alcohol and HRV.
After a hard workout
A hard interval session lowers HRV for a while, and harder sessions keep it lower for longer. A review of studies on recovery after aerobic exercise found that it took up to 24 hours after easy exercise, 24 to 48 hours after moderately hard exercise, and at least 48 hours after high-intensity work for the heart’s resting control to fully return, and that people with greater aerobic fitness recovered faster (Stanley and colleagues, 2013).
In the large app study, morning HRV was about 3% lower after a hard training day (Altini and Plews, 2021). A low reading on the morning after a tough session is therefore expected, and it is not a sign that something is wrong. The guide on HRV and training explains how some people use this pattern to plan their week.
When a cold is coming on
Illness tends to push HRV down and 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 up, often at the same time. In the app study, HRV was about 10% lower and heart rate about 6% higher on days when people were sick (Altini and Plews, 2021). A study of 2,745 Fitbit users in the US and Canada with COVID-19 confirmed by a PCR test found the same pattern, with breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate during sleep and heart rate going up while HRV went down (Natarajan and colleagues, 2020).
That does not mean your HRV can tell you that you are getting sick. In the COVID-19 study, a model built on these signals did better than chance at picking out the days when people were ill, but it was far from perfect: set to raise few false alarms, it caught fewer than half of the sick days. See HRV when you’re sick for what the research shows.
Stress, sleep and everything else
A hard week at work or a short night may also show up in your numbers. The authors of the app study described HRV as a more sensitive but not specific marker of stress (Altini and Plews, 2021). HRV reacts to many different things at once, and most of its limits, listed further down this page, follow from that. The guides on stress and sleep cover what studies have found.
Why understanding beats a score
Most apps turn HRV into a colour, a percentage or a “readiness” score, which is handy but hides the reasoning behind the result. Once you understand what sits underneath the score, you read the number in three new ways.
You stop comparing yourself with other people
HRV falls with age and differs a lot between healthy people of the same age. In the app study, age was moderately linked to HRV, while physical activity was only weakly linked to it. Age, sex, body mass index and activity level together explained only 15% of the differences in HRV between people (Altini and Plews, 2021). Someone with half your HRV may be just as healthy as you are, so the comparison that helps you is the one with your own normal range. The lesson on building your HRV baseline shows how to set one up.
You read single days calmly
One low morning after a hard session or a few drinks is not a warning. A drop that lasts several days without a clear reason is more worth a look, and knowing the difference keeps the number from running your mood.
You know when two numbers cannot be compared
Devices measure at different times and over different lengths of recording, and some report different measures. Values from a 24-hour recording, a 5-minute recording and a shorter one are not interchangeable (Shaffer and Ginsberg, 2017). If you know whether your app reports an average from the night or a single reading in the morning, you can judge whether a change is real. The lesson on why your devices disagree explains the differences, and the lesson on why higher HRV is not always better covers a common trap.
What HRV cannot tell you
HRV is useful because it reacts to so much, and that is also its main limit.
- It cannot tell you why it changed, because alcohol, illness, hard training, a bad night and stress can all push it the same way, so you have to supply the context yourself.
- It cannot diagnose anything. HRV does not tell you whether you have an infection, a heart condition or any other illness, and a normal reading does not mean you are well, just as a low one does not mean you are ill.
- It does not replace how you feel, and if you feel unwell, that matters more than any number on a screen.
- Published norms are not a verdict on you, because they depend on the method, the length of the recording and the people who were measured (Shaffer and Ginsberg, 2017).
Talk to a doctor if you have symptoms such as chest pain, fainting, a racing or uneven heartbeat, or shortness of breath, whatever your HRV says. The lesson on HRV limits, myths and red flags covers this in more detail.
How to start reading your own HRV
- Learn the basics first, because the opening lessons of the course make every number you see more useful.
- Measure the same way every time, at the same time of day, in the same position and with the same device. The lesson on how to take a good HRV reading walks you through it.
- Give it a few weeks, since you need a run of readings before you can see your normal range.
- Note what happened on each day, such as a drink, a late night, a hard session or a sore throat, and over time you will see which of them show up in your numbers.
- Watch the trend across a week rather than reacting to a single day.
Where to go next on hrv.expert
- The free HRV course takes you from what HRV is to reading your own trend, with a short quiz after each lesson.
- What affects HRV covers sleep, alcohol, stress, training, illness and more.
- Why does my HRV change so much? explains the day-to-day swings.
- What is a good HRV? puts your number in context.
- Is my HRV normal? checks a number from any device against published reference data.