What is HRV and Why Does it Matter for Sleep?

You might be familiar with HRV—heart rate variability—as a popular training tool for workouts, and a biomarker for measuring exercise recovery. Beyond exercise tracking, heart rate variability has a lot to tell us about the underlying health of our nervous system and our risk for disease. And there’s an important two-way relationship between HRV and sleep that’s often overlooked.

What is HRV, and why is it important?

Heart rate variability—HRV for short—is a measurement of variation in time between heartbeats. It is different from heart rate, or pulse, which is a measurement of the number of times the heart beats in a minute. But our hearts don’t beat at a fixed tempo. The intervals of time between heartbeats aren’t precisely the same every time—and HRV is a measurement of the changes in these intervals between heartbeats.

Let’s say your heart rate is 60 beats per minute (that’s on the low end of what is considered the normal range for a healthy adult). Looking at that number, it’s easy to imagine your heart beating once every second, for 60 seconds. In reality, the time between any two heartbeats changes—it might be 1.3 seconds followed by 0.9 seconds, and so on. This perpetual shifting is what we mean when we talk about heart rate variability.

Why does our heart rate shift in this way, from beat to beat? Heart rate variability is the result of the constant interplay of two parts of our nervous system, one that functions to stimulate our bodies to action and the other that works to reverse that stimulation and restore our bodies to a state of relaxation and calm.

The sympathetic nervous system revs the body to action, in response to stimuli in our external environment. It elevates heart rate and amplifies airflow through the lungs. It increases production of stimulating hormones such as adrenaline and cortisol. It elevates glucose in the bloodstream, and raises heart rate and blood pressure, increasing the supply of oxygenated blood throughout the body and to the brain. When the sympathetic nervous system is in high gear, it increases energy, sharpens alertness, focus, and reaction times, and powers major muscle groups.

At its core, the sympathetic nervous system activates our flight or flight response. It’s responsible for your pounding heart when a car swerves into your lane as you’re driving or when you have a big fight with your partner. It’s behind your rapid breathing when you’re about to speak to a crowd. It contributes to the buzzing, elated feeling when you’ve just gotten engaged, or been hit with some unexpected and exciting news. (Our fight-or-flight system responds not only to stressful and threatening situations, but also to positive stimuli from our environment.) The sympathetic nervous system also goes into high gear when we’re exercising and being physically active.

The parasympathetic nervous system counteracts the stimulation of the sympathetic nervous system, conserving energy, working to return the body to a state of relaxation and calm, and redirecting attention to functions that the body undertakes when it’s not consumed with taking immediate action. This is our rest and digest system: it lowers levels of stimulating hormones and increases calming hormones such as oxytocin; it slows heart rate and lowers blood pressure, relaxes muscles, slows breathing. The parasympathetic nervous system promotes digestion, elimination, sexual arousal, sociability, and sleep.

The sympathetic and parasympathetic systems are the two main components of our autonomic nervous system, which regulates essential involuntary physiological functions, including respiration, metabolism, digestion, elimination, fluid production (think: sweat, tears, saliva) and sexual response. The ANS—through the often-opposing forces of the sympathetic and parasympathetic systems—regulates heart rate and blood pressure, rate of breathing, and body temperature. Like the broader nervous system at large, the ANS is constantly working to respond appropriately to stimuli and to maintain balance, or homeostasis, in the body.

Heart rate variability is a result of both the sympathetic and parasympathetic nervous systems at work simultaneously in the body. Both systems are engaged all the time, taking on different degrees of dominance depending on the circumstances. The sympathetic nervous system pushes the heart rate faster. The parasympathetic system encourages the heart rate to slow down. It is this push-pull of these two sets of nervous system messages that produces the variation in time between heartbeats, aka HRV.

For this reason, HRV is an excellent measure of how well our autonomic nervous system is functioning.

A high HRV—meaning a wider variation of time intervals between heartbeats—indicates the body is responsive to both the sympathetic and parasympathetic systems, able to gear up into fight-or-flight mode AND to slow down into rest and digest, as needed.

A low HRV—shorter variation of time intervals between heartbeats–illustrates a lack of adaptivity between fight or flight and rest and digest, and is an indication of imbalance in the ANS, and is a likely indication that our fight or flight system is running in overdrive and over-dominant.

The resilience, flexibility and balance of our autonomic nervous system is critical to our mental and physical health. Imbalance and dysfunction between the sympathetic and parasympathetic systems is associated with inflammation, chronic stress, and chronic disease, including cardiovascular disease and type 2 diabetes, as well as depression and other mental health conditions. Low HRV scores can indicate elevated risks for heart attack and stroke, and are frequently present in people with depression and anxiety disorders.

Chronic dominance of the sympathetic nervous system—a body “stuck” in flight or fight mode—is also linked to sleep problems and sleep disorders, including insomnia and obstructive sleep apnea.

Let’s take a closer look at the relationship between the nervous system, HRV, and sleep.

HRV and sleep: a two-way street

The autonomic nervous system and sleep have a complicated, bi-directional relationship. The activity of our ANS affects our ability to fall asleep and sleeping through the night. How much and how well we sleep, in turn, affects the ability of our autonomic nervous system to function effectively, in everything from managing our stress response, to regulating cardiovascular activity, blood sugar and metabolism, and the regulation of hormones that affect mood and physiological function. Here are some key things to know about how the ANS works, and how HRV changes, during sleep:

• The parasympathetic and sympathetic systems are BOTH active during sleep, and their levels of activity shift throughout the night, during the transition from wakefulness to sleep and throughout the different stages of sleep.

• Falling asleep is associated with an increase in parasympathetic activity. No surprise here: we need to be in rest and digest mode to drift off into sleep.

• In stages of non-REM sleep, which include both light and deep, slow-wave sleep, the parasympathetic (aka rest and digest) system continues to predominate, with some fluctuations. Non-REM sleep is broadly associated with a higher HRV.

• During REM sleep, the sympathetic (aka stimulating) system becomes much more active. REM sleep is broadly associated with a low and variable HRV.

• Awakenings during the night—during ANY stage of sleep—increase sympathetic activity, elevate heart rate and blood pressure, and lower HRV.

Remember, we pass through the stages of non-REM sleep, and REM sleep, in every sleep cycle. And a typical 7-hour night of sleep contains between 4-5 sleep cycles. That gives you a sense of how complex and dynamic ANS activity is during sleep, with heart rate and heart rate variability continually shifting. It also demonstrates how important the flexibility and responsiveness of our nervous system is to a night of healthy sleep—and how restless and short sleep can throw our nervous system out of balance.

That imbalance is what occurs alongside disrupted, unrefreshing sleep, and in sleep disorders. And it’s not only an imbalance that occurs at night. Disordered sleep contributes to imbalance in the autonomic nervous system, and hyperactivity of the sympathetic nervous system, throughout the 24-hour day.

Insomnia and HRV

Research shows that people with insomnia experiencing increased sympathetic nervous system both during the day and at night. At the same time, parasympathetic activity is impaired during the day and night. With insomnia, people tend to spend more time in sympathetic (aka fight or flight) overdrive, with elevated heart rate and blood pressure, and a lower HRV. Deprived of sleep, the nervous system is less adept at shifting into rest mode, even at night and despite feeling exhausted.

This is the difficult cycle that people with insomnia experience, where poor sleep contributes to chronic hyperarousal of the sympathetic nervous system, which in turn leads to even more trouble sleeping. Being stuck in fight or flight mode is also one way insomnia can contribute to increased risks for cardiovascular disease (including high blood pressure, heart attack and stroke), type 2 diabetes, depression and anxiety—all conditions that are linked to hyperarousal of the sympathetic nervous system and a low heart rate variability.

Sleep apnea and HRV

The disordered breathing of sleep apnea—which comes from repeated episodes of partial or complete airway obstruction during sleep—deprives the body of sufficient oxygen and causes sleep to be fragmented with frequent arousals. Research shows that both the lack of oxygen (known as hypoxia) and the fragmented sleep of OSA lead to hyperarousal of the sympathetic nervous system, during sleep itself and during wakefulness. People with OSA tend to have lower heart rate variability during the day and while sleeping.

Sleep apnea is strongly associated with high blood pressure, insulin resistance and high glucose, with metabolic disease and type 2 diabetes, with elevated risks for heart attack and stroke and with mood disorders such as depression and anxiety. The imbalance of the sympathetic and parasympathetic systems (fight and flight vs. rest and digest) that goes along with sleep apnea appears to be a major contributor to those elevated disease risks.

Good news for people with sleep apnea: CPAP, the front-line treatment for OSA, has been shown to reduce sympathetic nervous system over-activity, and to improve parasympathetic function during sleep. With consistent, long-term use, CPAP reduces risks for cardiovascular problems.

How (and when) to measure HRV

So, you understand heart rate variability is a key measurement of the health and flexibility of your nervous system. And since your nervous system has a major hand in regulating nearly every aspect of your physiological, cognitive, and emotional functioning, HRV is a powerful signal of health and the body’s resilience—physical and mental.

How do you use HRV in your daily life, to improve health, create more energy, and better your sleep? HRV is a core measurement of fitness and health trackers. But you need to understand when to measure, and how to think about the results.

HRV changes throughout the day and depending on our circumstances and level of exertion. After a workout, for example, HRV will drop. That’s because exercise activates the sympathetic nervous system. Many people use HRV measurements after a workout to gauge post-workout recovery and avoid overtraining.

For broader health purposes, you want to establish a sense of where your heart rate variability is at in state of wakeful rest. I recommend getting an HRV reading first thing in the morning, after you’ve gotten out of bed but before breakfast and before coffee or any other stimulants. Whatever time you choose, make sure it’s a time when you will be relaxed, and not in the midst of or recovering from a bout of physical exertion (like a workout) or are carrying an actively heavy mental load (like in the middle of juggling a busy workday).

To get a reliable sense of where your heart rate variability is over time, take a measurement every day, or every couple of days, at the same time. This will give you a baseline to work with in improving your HRV, and keep you alert to positive and negative trends over periods of weeks and months.

Ways to raise your HRV

Good news here: Many of the most important habits that promote healthier sleep are also excellent for improving nervous system function, and helping to avoid chronic hyperarousal of your fight or flight system. If you’re tracking HRV and employing these practices regularly in your life, you’re likely to see your heart rate variability rising—and that’s a great sign for your sleep, your energy and mood, and your underlying, short- and long-term health.

Get more—and better—sleep. I can’t NOT put sleep first on the list here. A consistent routine of high-quality sleep can help your nervous system restore and maintain its balance, and remain flexible and adaptive to whatever circumstances and stimuli you encounter during the day. Sleeping well is associated with lower risks for cardiovascular disease, diabetes and metabolic disorders, and can help elevate mood and reduce risks for mood disorders. Sleep’s balance-restoring impact on our nervous system is one important factor.

Exercise regularly. Heart rate variability improves with physical fitness—and so does sleep itself. But that doesn’t mean you need to log long, grueling daily workouts to achieve your fitness and health goals. (Overtraining and pushing too hard, too often in workouts can backfire, leading to decreases in HRV.) In my latest book, Energize!, my co-author (Stacey Griffith, a founding instructor at SoulCycle) and I share a plan for daily movement that’s built around a series of short workouts that are personalized to your body type and chronotype. You can learn more about the Daily 5 x 5 here and check out the book here.

Practice mindfulness. Meditation, breathing exercises, and mind-body routines like tai-chi and yoga can help improve HRV , strengthening our body’s ability to move into rest mode and keeping our symptomatic nervous system in check. Mindfulness meditation practices also have powerful benefits for sleep. Start simple: pick a time for even one minute of mindfulness meditation every day and build from there.

Eat a balanced, healthy diet (on a schedule that fits your individual biology). Eating well contributes to better nervous system function and better sleep. When it comes to the particulars of your diet, there are plenty of healthful options these days that focus on whole foods and limit sugars and processed foods that mess with sleep and increase inflammation.

Keep this in mind: the WHEN of your eating is as important—if not more important—than what or how much you eat. In Energize!, we show you how to organize your daily eating routines to align with your specific body type and chronotype, to help you sleep better, give you consistently abundant energy, and promote physiological balance.

Be kind to yourself. Self-compassion is an unheralded superpower, when it comes to improving our health and resilience. Self-compassion on its own improves our resilience to stress and enhances our well-being—and it also has been shown to increase other healthy behaviors. As you’re building new habits, and tackling all that life throws your way, treat yourself with compassion and patience, the way you would a dear friend. It’s good for your heart and good for your sleep.

Sweet Dreams,

Michael J. Breus, PhD, DABSM

The Sleep Doctor

www.thesleepdoctor.com

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