The most important metric you're not tracking

What HRV tells us about our physical fitness and biological age

Hello to the 575 smart, motivated, and curious subscribers receiving today's email. And welcome if you've just joined us! Today's edition is a different one because I'm honing in on a very specific health-related topic. Usually, I'm a bit more focused on the mental models that can help us excel in all parts of our lives—our careers, our relationships, and our physical and mental health. But today... well, you'll see. Let’s get into it!

💡The big idea

Okay. I'm just going to be honest with you. I'm basically obsessed at this point. My poor wife. It's all she's been hearing about—for weeks. If I mention the term again, I might get thrown out. Seriously. It could happen.

But I can't stop thinking, reading, listening, talking—and now writing—about HRV. Yes, I'm obsessed with Heart Rate Variability.

Kind of a random thing to get obsessed with, you might think. And I wouldn't blame you for thinking that. I thought the same. Until I started going down the rabbit hole. Now I'm in deep.

Let me tell you about it. The best that can happen is that you might learn something. You might even get interested yourself. The worst that can happen is that you'll probably just end up doing my wife a solid by being another pair of ears (or eyes) for me to lay this on.

So, if you're game, let's dive in. But before we do, a quick disclaimer:

Please remember: I am not a doctor. But... I did stay at a Holiday Inn Express last night. So there’s that. Anyhow, let’s move on.

What is HRV?

Heart rate variability, it turns out, is appropriately named. It is the measure of the time between each beat of your heart—and more specifically, how that changes over time. We know, for instance, that our Resting Heart Rate (RHR) might be, let's say, 60. That means that when we are at rest, our heart is beating 60 times per minute on average. But unlike a metronome, the time between beats is not the same. Some beats may have 1 second between them, while others may have 1.5 seconds, and still others may have 0.5 seconds.

Why is this notable? Because it turns out that HRV is a phenomenal signal for telling us what is going on elsewhere in the body. Specifically, HRV gives us a window into how the autonomic nervous system is functioning—that's the part of our nervous system that controls involuntary actions (digestion, sweating, organ functioning, etc).

And even more specifically, HRV gives us a great indication of how balanced the two parts of our autonomic nervous system actually are.

Those two parts are:

  • The sympathetic nervous system — which controls our "fight or flight" response — basically, this is the system that sends a signal to organs throughout our body that we are in crisis mode—that we need to fight or run. It inhibits our digestion; it dilates our pupils; it speeds up our heart rate.

  • The parasympathetic nervous system — which controls our "rest and digest" response — this is the system that does basically the exact opposite; it sends the signal that all is well, and we can relax. It enacts digestion; it constricts our pupils; it slows down our heart rate.

A higher HRV reading tells us that our body is ready to move efficiently back and forth from one system to the other. A lower HRV reading, on the other hand, indicates that the signal from one system—almost always the sympathetic or "fight or flight" system—is dominating the other. It's an indication that our bodies are not using critical resources optimally and typically there’s a reason why that is the case.

Why is HRV important?

Two reasons: the short-term implications and the long-term ones. I'll take these one by one.

Short-term — A better understanding of how our bodies are currently performing

Our current HRV level is an effective measure of how ready our bodies are to handle stress. In other words, it's a great measurement for how recovered our bodies are from recent stresses that we have put on them. The fitness wearable company, Whoop, calculates its daily recovery score almost exclusively by measuring HRV (with small weights to other things like time asleep.) There's a reason you are now seeing athletes like Rory McElroy, Michael Phelps, and Patrick Mahomes wearing Whoop — it's because this measurement is an incredibly effective tool for helping elite athletes (and even newsletter writers like this one) understand what is happening within their bodies.

More specifically, tracking HRV over the short-term can be helpful in two ways:

  1. It gives us a very clear indication of the body's current capacity to take on stress—or in other words, how recovered we actually are. Interestingly, more trainers today are moving to workout regimes arranged not chronically but rather aligned with optimal recovery—or HRV—levels. Also of note is that any illness the body is battling is likely to impact HRV, in some cases, dramatically. In fact, if you track this measure regularly, you'll likely see any illness in your numbers before you actually feel sick. There have been plenty of examples of Whoop members identifying COVID-19 early by tracking HRV, respiratory rate, and a host of other metrics—including pro golfer, Nick Watney, before a recent tournament.

  2. Just as importantly, what this data provides on a daily basis is insight into how the lifestyle choices we make impact our recovery levels. For instance, HRV is extremely sensitive to alcohol. I have witnessed this personally when after a few glasses of wine, I watched my HRV plummet the next day. A Whoop study suggested that it takes four days for alcohol's impact on HRV to resolve.
    Note: Don't shoot the messenger.

In summary, it's somewhat shocking to me that we know more about the underlying performance of our phones, cars, and computers than we do about our own bodies. HRV is one critical measurement that helps to close this gap. In this regard, following the fluctuations in one's HRV can be incredibly informative, particularly when they can be mapped to variables from alcohol, to hydration (which also has a large impact), to exercise, nutrition and sleep.

Long-term — The best overall marker of biological age?

While the short-term measurement of HRV can be a powerful source of information and motivation, thinking about HRV as a long-term measure of health can be even more impactful. What can HRV tell you about long-term health? A lot, actually. Research has shown low HRV to be associated with a dizzying number of both physical and mental conditions—but it appears to be particularly associated with worsening depression and anxiety, as well as an increased risk of cardiovascular disease, and even death.

Dr. Bob Arnot summed it up well when he said that while HRV is a great indicator of day-to-day recovery, it may be "the best overall marker of biological age."

Measuring HRV

Okay, so if I’ve convinced you that HRV is an important measure to track, the next logical question is: How the heck do I track it?

The good news on this front is that as HRV has gained increased acceptance as an important health metric, the market has responded by providing a number of consumer-friendly devices that track (and even interpret) HRV readings for you. The ones most common at the mass market level today are the Apple Watch, Whoop, and Oura ring. Each has its own pros and cons but any one of them (or in my case, two) will give you a great start on monitoring your HRV.

So where can you find this info and what does it look like?

If you have an Apple Watch, go to the Health app on your phone, which looks like this:

And then navigate to Heart Rate Variability. You should be able to see data going back as far as you've been wearing the watch. If you scroll down on the Heart Rate Variability page, it gives you an option to add it to Favorites. Do that and then it will appear on the first page whenever you open the app going forward.

If you have a Whoop, you'll notice that there is a bit more intentional focus on HRV. One of the most addictive parts about Whoop is the Recovery score you get every morning when you wake up. As I said previously, it's largely based on your HRV compared to your own baseline level. The level of data and reports available with Whoop is impressive but here's just a quick screenshot to give you a sense.

What is a “good” HRV?

As far as I can tell, no one wants to answer this question. And I think that it’s largely because HRV is such a highly personalized metric. There are many factors that influence your HRV level—some are lifestyle-oriented—like how physically active you are, your tendency to consume alcohol, your sleep habits, etc.—while others are more fundamental like our age group (HRV declines pretty dramatically with age) our gender (women tend to have lower HRVs) and our genetics.

But I’m not just going to leave you hanging like that. Below are a few interesting charts that Whoop has published from their own users’ data to give you an indication of where people’s HRV values fall.

This is users age 20 to 65; I’d note that a typical Whoop member is likely more physically fit than the general population—so don’t get too discouraged if you fall below this range.

How to Improve HRV

Okay, so we know what it is. We know why it’s important. We know how to track it. And we even have a vague idea of what might be “good” and “bad” numbers. So what should we do about it?

Well, I might suggest that we try to improve it. To do so, we’ll need to think both long-term and short-term.

The list of action items to improve your HRV is probably not going to shock you, mainly because it looks like the list of action items to improve your health overall. But here goes anyhow…


  • Rest / Sleep — a low current HRV reading may be telling you that you’re run down. Rest.

  • Get over illness — as stated above, illness shows up quite obviously in this data.

  • Hydrate — if your urine is yellow, you’re not drinking enough. Drink water, pee clearly, and watch your HRV rise.

  • Avoid alcohol — again, don’t shoot the messenger.

  • Avoid sensitive foods (highly personal) — best to experiment to see what foods impact you.

  • Meditation / stress reduction — stress makes HRV plummet; you’ll believe in meditation as soon as you witness the tangible proof in your own data.


  • Cultivating an exercise habit — balancing exertion with rest.

  • Dialing in nutrition — make good choices, see the benefits in your numbers.

  • Addressing mental conflicts (lack of purpose, wrong career, etc.) — feel like I need a deeper dive on this… hmm, next article, maybe?

  • Psychological (talk) therapy — the mind/body connection is real, and it’s fabulous.

  • Building a meditation or mindfulness habit — Just do it!

  • Improving breathing — I need to learn more on this one myself but my basic understanding is that we need to focus more on the exhale.

HRV — Gamifying your health

What’s most intriguing to me about HRV is that not only is it a phenomenal measure of both short-term and long-term health, but also, it’s a measure that we can actually do something about.

We all know that we should lose weight or get in better shape or reduce stress. But we also know that life gets in the way. And we tend to put worrying about our health off to another day.

I’m a huge believer in “what gets measured gets done” and in streaks. You know I love to gamify anything I can to help modify my own behavior. HRV is perfect for this. Not only can I monitor my long-term trends and get an indication on whether or not my overall health is improving or deteriorating, but I’m also getting daily feedback on how I’m doing vs. yesterday, last week, or last month.

This is important because it puts me in a competition with myself. A lot of what I write about in this newsletter is motivation and finding strategies and tactics to help to motive us to do the things we already know we should be doing.

Tracking HRV, at least for me, has quickly ascended to the top of that list.


That's it for this week but before you go I wanted to give you some resources on where to learn more. So check out the Content Diet section below.

By the way, I’m not sponsored by Whoop—just a fan and a customer. But if you want to check out Whoop, here’s a referral code to get you a free Whoop + 1 month free.

Have a great week.


Content Diet

As I mentioned earlier, I’ve been reading and listening to a lot on HRV lately. Here are some of the best resources I’ve found.

🎙 Whoop podcast — from guess who? Whoop!
HRV 101: Insights from the Whoop Podcast — This is a great starter episode covering why HRV is such a critical metric. It features Rory McElroy, Steve-O, and others.

HRV: What it is, how to improve it and what it means to recovery and human performance — this is a little deeper dive into all of the above.

🎙 Elite HRV podcast — these are more in-depth discussions with experts and are super insightful.

How to Shift Out of Survival Mode into Growth — great conversation on the mind/body connection and how stress and HRV are related

Mental Health, HRV & Antidepressants — a deeper dive into mental health and its connection with HRV. Incredible to see the connection with depression

Lots and lots more has been written and recorded on the subject so if you’re interested, you’ll be able to find plenty. And if you find something particularly intriguing on the topic, please share it with me because as you can tell, I’m a little bit obsessed with it at the moment. I repeat: My poor wife.

See you in two weeks.

— Greg