Heart Rate Variability (And The Coffee that Almost Killed Me)
This is the ultimate heart rate variability deep dive—nuanced, practical, and funny. I'll even share a story involving Andrew Huberman, Starbucks, and my girlfriend’s friend seeing my butt. Really...
Heart Rate Variability (HRV) might be the most accessible—and powerful—predictor of cardiovascular and nervous system health that you are probably not measuring. Today’s letter is dedicated to decoding what it is, how to measure it, and how to change it.
But first, some framing. When I started digging into the historical literature on heart rate variability, I was blown away—not just by how good it was, but by how deep it went.
Here are some fast facts to warm up your brain:
Even after adjusting for confounders, low HRV is associated with ~50% increase in risk of a having a heart attack.
And, in those who have had a heart attack, risk of death is 5X higher in those with low HRV compared to those with higher HRV.
We’ll unpack these data in more details shortly. But that gives you a sense of the stakes.
But now, let’s pause. Take a deep breath, and explain what is HRV is anyway?
What is Heart Rate Variability (HRV)?
HRV is not your resting heart rate—the number of beats per minute (bpm)—but the variability within those beats. Because the reality is, your heart doesn’t beat like a metronome.
For example, at 60 bpm, one interval might be 1.0 seconds, the next 1.05, the next 0.92, and so on.
HRV captures these microfluctuations in the lub-dub in your chest: lub-dub. luub-dub. lub-duub. lub—dub.
And, those microfluctuations are a direct signature of what your nervous system is doing to your heart.
Here’s the physiology in plain English, or as plain as I can make it.
Your autonomic nervous system has two main branches: the sympathetic (“fight or flight”) and the parasympathetic (“rest and digest,” largely mediated by the Vagus nerve). Both act on the heart’s pacemaker—the sinoatrial node.
The sympathetic system pushes your heart to beat faster and more steadily.
The parasympathetic system does the opposite—it slows things down. And, it does so rapidly and dynamically, creating variability from beat to beat.
Okay—still with me?
Let’s make it intuitive.
Think of your heart as the engine of your body, which it is.
The sympathetic system is like pressing on the gas: you speed up, and the ride becomes smooth and steady—low variability.
The parasympathetic system is like tapping the brake: small, rapid adjustments that create subtle fluctuations in speed.
Now, the analogy fails a bit here—because when you’re driving, you don’t want someone tapping the brakes. But in physiology, that’s exactly what you want. Those tiny adjustments reflect a brake system that’s responsive, precise, and working well.
You don’t want broken brakes.
Seen this way, HRV is a marker of vagal tone—your ability to access “rest and digest,” or more simply, recovery.
And zooming out, it reflects something even broader: your body’s capacity for adaptive regulation—the ability to continuously adjust to internal and external demands.
High HRV = a flexible, responsive system with strong parasympathetic control.
Low HRV = reduced vagal tone, sympathetic dominance, and a system that’s less adaptable—more like flying down the highway with broken brakes.
So, hear me when I say this: HRV isn’t just a cardiac metric.
HRV is an emergent property of brain–body integration—a real-time index of how well you maintain homeostasis in a constantly changing world.
Oh, sh*t. Fido, no don’t eat that…
…Ugh. Okay, I’m back. But I need a new wallet.
Where was I?
Anyway, that’s what HRV is. Now comes the why and the how(s).
Today, we’ll walk through the key papers showing why HRV matters—and then get practical: how to measure it and how to improve it. Specifically, we’ll cover data showing:
HRV predicts death.
HRV predicts your first heart attack.
How to measure HRV
A practical, science-backed guide to improve HRV
Specific Exercise Suggestions
A Nutrient that Increases HRV
Specific Biofeedback Protocols
And I’ll even share the craziest Starbucks story you’ve ever heard. 100% guaranteed. It features a massive tech outage, Andrew Huberman, and my girlfriend’s friend examining my butt. And it ties back to HRV. I promise.
Like all our best deep dives, this letter is for StayCurious Metabolism Premium members. In an ideal world, everything I create would be free—but building high-quality, multi-media content and pushing metabolic health into the mainstream takes resources. Premium members make that possible. (A huge thank you!)
Please consider joining our community—and investing in your long-term health, learning, while also supporting our mission to make metabolic health mainstream, for less money per month that a single Starbucks run. But I also guarantee, this letter alone will be well worth that $8 monthly premium subscription.
