Heart Rate Variability (HRV): What It Is and How to Use It

By now, many people have heard of heart rate variability. It shows up on our wearables, recovery tools say they can improve it, and some friends may even brag about how high theirs is. To use that information wisely, we first need to understand what HRV is and why it matters.

What is heart rate variability (HRV)?

Simply put, heart rate variability (HRV) is the variation in time between consecutive heartbeats. That may sound odd, since we usually see a heart rhythm drawn on an electrocardiogram (ECG) like this:

On standard ECG paper, each large box represents 0.20 seconds and each small box represents 0.04 seconds. Even when the rhythm looks regular, the precise time between each heartbeat varies slightly from one beat to the next.

How breathing changes your heart rate

When we breathe in, pressure in the chest changes and heart rate speeds up a little. When we breathe out, it slows again.

Part of the reason is that vagus nerve activity decreases while we inhale. This is called respiratory sinus arrhythmia. It’s a normal, healthy rhythm and one of the factors that contribute to your HRV.

The vagus nerve and your “rest and digest” system

The vagus nerve is a major pathway through which the parasympathetic nervous system (PNS) influences the heart, as well as many other organs.

The PNS is like your body’s calm-down switch. If the sympathetic nervous system (SNS) is your anxious older sister hosting Thanksgiving, your PNS is the uncle dozing in the recliner after the turkey. It’s the “rest and digest” system that slows your heart rate and supports digestion and energy storage, which is why we need it to recover after activity.

Importantly, HRV is not simply a measure of fitness or “how healthy you are.” It’s better viewed as a dynamic marker of how your body is responding to what’s being asked of it right now. During exercise and other everyday stressors, the SNS turns on: heart rate and breathing rate go up, and blood flow to the gut goes down. As heart rate rises, HRV falls.

How easily your body can shift back into this rest-and-recover mode tells us about more than fitness. It’s also linked with long-term heart health.

Why does HRV matter for your health?

HRV gives useful information about how your brain, autonomic nervous system and cardiovascular system work together, and how well your body responds to stress.

HRV’s first clinical use was in pregnancy, believe it or not. In the 1960s, researchers noticed that changes in the timing between a baby’s heartbeats could signal fetal distress before the heart rate itself changed.

In the 1970s and 1980s, reduced HRV was linked with higher risk after a heart attack and with nerve damage from diabetes [1]. In people without known heart disease, lower HRV has been associated with a higher risk of a first cardiovascular event [2]. Since then, HRV has been applied far more broadly, in both clinical and healthy populations.

One of the most common uses of HRV is as a window into recovery: how your body’s balance shifts in response to internal or external stressors.

Why your HRV drops, and what it doesn’t mean

A lower HRV doesn’t automatically mean you’ve lost fitness.

The body is constantly balancing stress and recovery. Exercise is a stressor, but so are poor sleep, psychological stress, illness, dehydration, alcohol, travel, eating too little, heat exposure and not enough recovery between workouts.

For example, say someone’s morning HRV is normally fairly stable. Then it drops substantially for several days in a row, their resting heart rate rises, and they report poor sleep and fatigue. Together, those signs may point to increased physiological strain.

That doesn’t mean their long-term fitness, such as VO2 max, has declined. Their performance may be lower on a given day because of the extra stress and poor recovery, but the underlying adaptations are still there.

Is a higher HRV always better?

Not always. HRV varies a great deal from person to person and is influenced by age, genetics, fitness, sex, body position, breathing and how it’s measured [3]. Two healthy people can have very different HRV values.

A reading that’s unusually high or jumps around can also come from irregular heartbeats, such as extra beats or atrial fibrillation, rather than better recovery. If your device flags an irregular rhythm, talk with your physician.

That’s why the most useful information comes from your own HRV trend over time, not from comparing your number with someone else’s.

How to measure HRV accurately

The key words here are consistency and patience.

HRV is extremely sensitive to measurement conditions. Comparing a reading taken in bed right after waking with one taken after exercise or a cup of coffee can produce big swings and misleading information.

For tracking over time, try to measure HRV:

• At about the same time each day
• In the same body position
• After waking, and before exercise
• Before caffeine when possible
• Using the same device and measurement method
• With fairly consistent breathing and surroundings

I also like to keep a log of any change in my normal routine within 24 hours of a measurement, for example traveling to a new time zone or having wine with dinner.

Overnight measurements can also be useful because they collect a lot of data and avoid some of the variability of a single spot reading.

How accurate are wearables for HRV?

An ECG or a chest-strap heart rate monitor gives the most accurate HRV reading. Wrist and finger wearables estimate HRV from your pulse, using light sensors that detect blood flow under the skin. They’re useful for tracking patterns over time, but accuracy varies by device [5].

They’re most reliable when you’re still, such as overnight or right after waking, and least reliable when you’re moving [4]. That’s one more reason to compare readings taken under the same resting conditions, on the same device.

Devices also report different HRV measures. Apple Watch, for example, reports a measure called SDNN, while many rings and straps report one called RMSSD, so numbers from two brands aren’t directly comparable.

What is a good HRV? Start with your own baseline

There is no single HRV number that everyone should achieve.

The most practical approach is to establish a personal baseline:

1. Use your tracker or wearable for at least one month before changing any of your lifestyle habits.
2. Look for patterns rather than individual readings, and cross-reference them with lifestyle factors like the ones above.
3. Change only one or two habits at a time, and give each one 3 to 4 weeks to see how your average HRV responds.
4. Don’t stress if it moves in the “wrong” direction. Fluctuations are normal and will happen as life happens.

How to use HRV to guide your training

Always take HRV in the context of everything else going on. Here’s how that looks in practice.

You traveled, had a few drinks on vacation, and now your HRV is lower. This may not be a sign of deconditioning. Instead, it may be a sign to swap hard workouts for moderate ones and work on the other factors that affect HRV. Get back on track with your sleep and normal diet, and see how that goes before bumping your workout intensity back up.

Nothing changed, you feel good, but your HRV is a few points lower than usual. No stress! Take it as one data point and listen to how your body feels. Address it if other symptoms show up, like fatigue, fever or a drop in performance.

You’re training for a century ride, with more long rides in the heat and humidity. If you’ve felt less motivated to work out and your HRV has stayed below your average for more than a couple of days, that’s a sign you need a recovery day to avoid overtraining.

Your HRV is chronically low. Let’s look at your exercise patterns. Are you getting at least 150 minutes a week of moderate-intensity cardiovascular exercise [6]? If not, steadily add about 10 minutes a week until you get there.

In studies, regular exercise training tends to raise HRV over a few months, though everyone responds differently [7]. From there, you can add short bouts of intervals to build on your training. If your HRV stays low despite these changes, talk with your physician, since some medical conditions and medications can also lower HRV.

A simple guide for adjusting your workout

How your PrimaryMD care team uses HRV

For PrimaryMD members, wearable data is one input, not the whole picture. Your physician-led care team, including your exercise physiologist, can review your HRV trends alongside your sleep, training, labs and history, and help turn a number on your watch into a decision you can act on.

The bottom line: chase adaptability, not a number

Don’t chase a higher HRV. Chase better adaptability.

A healthy body should be able to respond to stress when it needs to and return toward its normal state once the stressor is gone. HRV can help us observe part of that process. Note it, take it in the context of the rest of your life, and use it as a tool, not as a standalone number to correct.

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