By: Michael Beiter

Heart rate variability (HRV) measures how well our autonomic nervous systems work together to help us respond to situations. Every time we exhale, our parasympathetic nervous system sends a mild inhibitory “calm-down” signal to our heart via the vagus nerve, which slightly slows our heart rate while we’re exhaling. As soon as we inhale, this signal is dropped, and our heart rate switches back to a slightly faster rate. This back-and-forth oscillation produces what is known as heart rate variability (HRV).

Having a high heart rate variability tends to mean a more responsive and healthy autonomic system. This means that both parasympathetic and sympathetic nervous systems are adapting to stress.

But should we track HRV?

Is HRV always useful and important to know?

The answer is not straightforward.

When it comes to athletic performance, there is some evidence that HRV biofeedback training may improve fine and gross motor performance, but the results are mixed. Although HRV gives us a window into our stress responses, it’s also not the only factor. Heart rate (HR), for example, often relates to HRV (e.g., if we start exercising, our HRV will go down while our HR goes up), but it still varies independently. For example, while at rest, you could have either high or low HRV at the same heart rate.

For “regular folks” just trying to feel and function better, tracking HRV for a while can be a useful tool to help learn how various stressors affect your body. Whether we’re stressed because somebody stole our credit card information online, we’re coming down with a cold, or we just did a tough workout, it will have an impact on our autonomic nervous systems and, in turn, our morning HRV measurements.

Likewise, when we recover better, we can also see this reflected in the data. If we get a great night of sleep, take up mindfulness meditation, or successfully finish a challenging work project, that will also affect our HRV.

HRV isn't as simple as "good" or "bad" HRV. Rather, it's only useful in context. There are times when you want your autonomic nervous system (ANS) primed for intense effort and engagement. And there are times when your nervous system should be shifted toward deep recovery and rest. HRV can provide a good window into what is happening within your body and mind. But in order to interpret this information, you have to put it into the context of what the ideal level of autonomic arousal is in that particular setting or point in time.

For most people, HRV values at rest are the most useful thing to observe. These reflect how our autonomic nervous system is managing our combined levels of mental, emotional, and physical stress. Recall earlier in this chapter when we talked about how anxiety is a response to the anticipation of a potential threat. This anxiety has a physiological fingerprint within our autonomic nervous system, and this is reflected in our HRV. So, even outside of physiological stress (like daily intense exercise), our HRV levels at rest can tell us how much chronic stress and anxiety we’re under. It's generally best to have high HRV at rest when we're in relaxation and recovery mode.

As people become more fit, they have higher HRV at rest. This can result from many factors — from the direct physiological effects of improved aerobic function to the mental and emotional benefits of exercise. But building stress management skills still helps.

HRV-based biofeedback or stress management practices that reduce chronic stress levels at rest can improve recovery and facilitate athletic performance and adaptations even more. This, in turn, further improves HRV. It’s a win-win.

During most forms of exercise, people will have low HRV. When we move strenuously and intensely, the sympathetic tone of the heart goes up, and the parasympathetic tone goes down. If our heart rate goes over about 100 beats per minute (bpm), we’ll naturally lose heart rate variability. This helps us “amp up” to do difficult, effortful tasks — like helping your buddy move the couch up three flights of stairs or paddling your surfboard frantically away from the hungry shark.

Regardless of the HRV reading, athletes still need to control their stress responses and regulate their arousal levels as they train and compete. The more “low” (i.e., parasympathetic activity) you can produce on demand, the more “high” (i.e., sympathetic activity) you can produce later. So, the ability to recover between efforts (such as between sets of a tennis match or rounds of an MMA fight) affects how much high intensity you’re able to sustain during the active portions of the match.

In conclusion, HRV can provide valuable insights into how our autonomic nervous system manages stress, recovery, and performance, but it's important to interpret the data in the context of the situation and individual needs. For most people, tracking HRV at rest can be a useful tool for managing chronic stress and anxiety levels, building stress management skills, and improving recovery. For athletes, HRV biofeedback training may improve performance, but it's not a performance enhancer in isolation.

Regardless of your goals, HRV can be a helpful gauge on the dashboard of your body's engine.