What Is Heart Rate Variability and What Does It Say About Your Sleep?

If you have an activity tracker, you’re likely familiar with many of the measurements used to gauge your health and exercise intensity—for instance, your resting heart rate or target heart rate and your overall activity score. Another metric that these devices provide is your heart rate variability (HRV)—a number that’s often a source of confusion even for elite athletes but is worth understanding.

Once you do understand HRV, it can provide useful information about your health, resilience and behavioral flexibility. Because HRV is closely tied to how well you rest and recover overnight, factors like sleep habits, stress and sleep environment—including comfort and support—can all influence your HRV during sleep.

HRV refers to the variation in time between the beats of your heart. So, for example, if your heart rate is 60 beats per minute, you might think that it means that your heart is beating once per second. But your heart is not a metronome. There are tiny variations (as tiny as a fraction of a second) in the amount of time between successive beats, which could mean that there is a gap of 8/10 of a second between two beats, for instance, and 1.10 seconds between two others.

HRV measures those specific fluctuations in time between your heartbeats. Though those variations are measured between heartbeats, they’re actually signals from your nervous system.

Your heart rate takes direction from your autonomic nervous system (ANS), which regulates not just how fast your heart beats, but also your blood pressure, breathing, digestion, even blinking your eyes—all the things your body does without your having to think about them.

“The ANS has two main parts,” says neurologist and Sleep.com Sleep Advisor Dr. Chris Winter, author of “The Sleep Solution.” “The sympathetic nervous system, often called the fight-or-flight response, responds to stressors (including exercise) by increasing your heart rate and blood pressure. The parasympathetic nervous system, known as the rest-and-digest or relaxation response, slows down your heart rate and blood pressure when you’re feeling calm and relaxed, or you’re sleeping.”

The part of the ANS that is activated depends “on what you’re doing and the messages your heart is getting from your brain,” Winter explains. When you’re snoozing under a beach umbrella or absorbed in a good book in a hammock under a shady tree—or you’re sleeping—you’ll have a slower heart rate, controlled by the parasympathetic nervous system.

On the flip side, if you’re feeling threatened, in danger or anxious—or you’re pushing through a tough workout—your heart rate will quicken, among other nervous-system responses, activated by the sympathetic nervous system. Once the stressor is gone, the parasympathetic nervous system kicks back in, telling your heart it’s okay to slow down and the rest of your body to relax and go back to doing its usual things.

“Heart rate variability reflects your body’s ability to manage the signals from these two competing branches of your nervous system,” Winter says. “When those systems are well balanced, there’s a lot of variability because your heart is being responsive to both parts of your nervous system. That’s why, generally speaking, a higher HRV is good: It means your body is better able to react to stress, and then bounce back once the stress is gone.”

In a medical setting, an electrocardiogram machine (EKG) is the most accurate way to detect heart rate variability. But these days there are non-medical wearable devices that can track your HRV and clue you in on an app or wearable device.

Winter finds that the most accurate readings are done while you sleep, when there are no environmental stimuli to react to or cause stress. Even if you’re awake but relaxed, your alertness, even if it’s as simple as thinking through a to-do list, can cause fluctuations that impact the variability. If you’re in flight-or-flight mode and your heart is racing, there won’t be much time between beats and your HRV will tend to be lower. When you’re relaxed, your heart will beat slower, so there’s more time between beats for variations.

But HRV can even change over the course of your sleep. Heart rate variability has been shown to decrease during the rapid eye movement (REM) phase of sleep. Even more, the intensity of dreams can impact variability, which can help sleep scientists better understand the physical impact of sleep cycles. Tracking your HRV for several weeks can help you determine your baseline.

A “normal” HRV for adults (typically measured in milliseconds) can range anywhere from below 20 to above 200, depending on your age and gender, among other things. “The important thing to know is that HRV varies from person to person,” says Winter.

“It’s a super-sensitive measurement, and can fluctuate a lot throughout the day, depending on your stress levels, your hydration, your sleeping patterns, as well as changes in your emotions and activities. And not only will it fluctuate during the day, but also from one day to the next.”

HRV levels naturally decrease with age. Younger people tend to have higher HRV than older people, and biological males often have slightly higher HRV than biological females. And elite or professionally trained athletes often have the highest HRV. But these are generalizations and not set in stone.

What constitutes a healthy HRV differs for everyone. If your friend has a higher HRV than you do today, that doesn’t necessarily mean they’re more fit or less stressed out than you are.

Rather than comparing your number to anyone else’s, “it can be helpful to track your HRV over a couple of weeks and follow your own long-term trends,” says Winter. “If, for instance, you’re taking steps to improve your overall health, over time you’ll likely begin to see an increase in your average heart rate variability.”

A lower HRV often means that your sympathetic nervous system is dominating and sending stronger signals to your heart than your parasympathetic nervous system. If your nervous system is in more of a fight-or-flight mode, the variation between consecutive heartbeats tends to be lower.

A downward trend in your HRV over a couple of days is worth paying attention to as it indicates that, for some reason, your body is in overdrive. It could be a sign of a problem, such as that you’re sleep-deprived, fighting a virus or not managing your stress. “If you’re not working out or doing something active, a low HRV—low for you—could mean your body is working hard for some other reason: maybe you’re fatigued, dehydrated, stressed or sick and need to take a few days to rest and recover, and get more good-quality sleep,” Winter says.

Studies show that low sleep HRVs may indicate a sleep disorder such as obstructive sleep apnea, so if yours is consistently low, consider the factors that may affect it: stress levels, bedtime routine, sleep environment and certain medications or medical conditions. If you think you may have a sleep disorder, talk to your doctor.

HRV can also give insights into your lifestyle and help motivate you if you’re thinking about taking up some healthy behaviors. You might see your HRV gradually increase as you incorporate more mindfulness, meditation, restful sleep and physical activity into your daily (or nightly) routine. If you’re someone who loves data, it’s a way to track how your nervous system is responding not only to your environment, but also to your emotions, thoughts and feelings.

There are a few different ways that you can improve your heart rate variability. Regular exercise and a healthy diet are good places to start, but mental health is also a key component of HRV. Reducing or managing your stress level can help keep that fight-or-flight response from dominating, which can improve your HRV. If you have concerns about anxiety or depression, a health care provider can help you figure out tools for managing it.

Here are some other ways to reduce stress and improve your health:

• Practice meditation 
• Start a gratitude journal 
• Practice deep breathing 
• Spend time outdoors 
• Get natural sunlight first thing in the morning 
• Listen to music you like 
• Spend time with family and friends 
• Drink alcohol in moderation, and not late at night 
• Get plenty of healthy, restful sleep 

Because heart rate variability is most accurately measured during sleep, anything that disrupts or supports stable, restorative sleep can influence HRV. A mattress that promotes proper spinal alignment, reduces pressure points and minimizes tossing and turning can help limit nighttime awakenings and support smoother transitions between sleep stages. By encouraging longer stretches of deep and REM sleep and reducing physical discomfort that activates stress responses, the right mattress may help the nervous system remain in a more relaxed, recovery‑focused state overnight, which can be reflected in healthier sleep HRV patterns over time.