What is HRV and Why Does it Matter for Sleep?

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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.

HRV 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, 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.

Our heart rates shift as a result of the constant interplay of two parts of our nervous system.

In response to stimuli in our external environment, the sympathetic nervous system elevates heart rate and amplifies airflow through the lungs. At its core, the sympathetic nervous system activates our flight or flight response. It increases production of stimulating hormones such as adrenaline and cortisol, elevates glucose in the bloodstream, and increases 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.

The parasympathetic nervous system counteracts the stimulation of the sympathetic nervous system, conserving energy and working to return the body to a state of relaxation and calm. This is our rest and digest system. It lowers levels of stimulating hormones and increases calming hormones such as oxytocin. it also slows heart rate, lowers blood pressure, relaxes muscles, and slows breathing.

Through the often-opposing forces of the sympathetic and parasympathetic systems, the autonomic nervous system regulates heart rate and blood pressure, rate of breathing, and body temperature. Your nervous system at large is constantly working to respond appropriately to stimuli and to maintain balance in the body.

Heart rate variability is a result of both the sympathetic and parasympathetic nervous systems at work simultaneously in the body. It is this push-pull of these two sets of nervous system messages that produces the variation in time between heartbeats. For this reason, HRV is an excellent measure of how well our autonomic nervous system is functioning.

• High HRV: This means a wider variation of time intervals between heartbeats and 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.
• Low HRV: A shorter variation of time intervals between heartbeats illustrates a lack of adaptivity between fight or flight and rest and digest. It may be an indication of imbalance in the nervous system and is a likely indication that our fight or flight system is over-dominant.

The resilience, flexibility, and balance of our autonomic nervous system is critical to our mental and physical health. Imbalance 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. Dominance of the sympathetic nervous system 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.

The autonomic nervous system and sleep have a complicated, bi-directional relationship. The activity of our nervous systems 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 hormones.

The parasympathetic and sympathetic systems are BOTH active during sleep, and their levels of activity shift during the transition from wakefulness to sleep and throughout the different stages of sleep. Falling asleep is associated with an increase in parasympathetic activity (aka rest and digest). In stages of non-REM sleep, the parasympathetic system continues to predominate and is broadly associated with a higher HRV, with some fluctuations. During REM sleep, the sympathetic (aka stimulating) system becomes much more active and is associated with a low HRV.

The flexibility and responsiveness of our nervous system is critical to a night of healthy sleep—and restless or short sleep can throw our nervous system out of balance. That imbalance can continue 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.

Being stuck in fight or flight mode is also one way insomnia can contribute to increased risks for cardiovascular disease, 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 partial or complete airway obstruction during sleep, deprives the body of sufficient oxygen and causes frequent arousals. Research shows the fragmented sleep of sleep apnea leads to hyperarousal of the sympathetic nervous system, during sleep itself and during wakefulness.

Sleep apnea is strongly associated with high blood pressure, insulin resistance and high glucose. The imbalance of the sympathetic and parasympathetic systems that goes along with sleep apnea appears to be a major contributor to those increased health risks. Fortunately, CPAP has been shown to reduce sympathetic nervous system over-activity and to improve parasympathetic function during sleep.

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 the body’s resilience—physical and mental.

HRV is a core measurement of fitness and health trackers. As it changes throughout the day, depending on our circumstances and level of exertion, 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, coffee, or any other stimulants. Whatever time you choose, make sure it’s a time when you will be relaxed.

To get a reliable sense of where your heart rate variability is over time, take a measurement every day, 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 time.

Many of the most important habits for better sleep are also excellent for improving nervous system function. 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, energy, mood, and overall health.

• Get better sleep: 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 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.
• Exercise regularly: Heart rate variability improves with physical fitness—and so does sleep itself. But that doesn’t mean you need long, grueling daily workouts. In fact, overtraining and pushing too hard, too often in workouts can backfire, leading to decreases in HRV.
• Practice mindfulness: Meditation and breathing exercises can help improve HRV, strengthening our body’s ability to move into rest mode. Mindfulness meditation practices also have powerful benefits for sleep.
• Eat a balanced, healthy diet: Eating well contributes to better nervous system function and better sleep. There are plenty of healthful options these days that limit sugars and processed foods. Keep this in mind: WHEN you eat is as important as what or how much you eat.
• Be kind to yourself: Self-compassion on its own improves our resilience to stress and enhances our well-being. It also has been shown to increase other healthy behaviors. As you’re building new habits, and tackling all that life throws your way, remember to have compassion and patience. It’s good for your heart and good for your sleep!

About The Author

Dr. Michael Breus

Clinical Psychologist, Sleep Medicine Expert

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Michael Breus, Ph.D is a Diplomate of the American Board of Sleep Medicine and a Fellow of The American Academy of Sleep Medicine and one of only 168 psychologists to pass the Sleep Medical Specialty Board without going to medical school. He holds a BA in Psychology from Skidmore College, and PhD in Clinical Psychology from The University of Georgia. Dr. Breus has been in private practice as a sleep doctor for nearly 25 years. Dr. Breus is a sought after lecturer and his knowledge is shared daily in major national media worldwide including Today, Dr. Oz, Oprah, and for fourteen years as the sleep expert on WebMD. Dr. Breus is also the bestselling author of The Power of When, The Sleep Doctor’s Diet Plan, Good Night!, and Energize!

• POSITION: Combination Sleeper
• TEMPERATURE: Hot Sleeper
• CHRONOTYPE: Wolf

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