Heart Rate Variability (And The Coffee that Almost Killed Me)

This coffee almost killed me. But in so doing, it provoked my profound curiosity for the interconnectedness of our nervous systems and our hearts, and one biomarker in particular that can predict a 5-fold increased risk of death. If you aren't measuring it, you should be. It's called heart rate variability. Heart rate variability, or HRV for short, can predict death in some, increased risk of first heart attack in others, but it's so much more than just a heart health marker. It's a marker of effective brain-body integration. You could say neurocardiac agility. And it's easy to leverage and easy to modify to improve your overall health. I'm gonna show you. And I promise we will return to that horrible day where I nearly killed myself with a cup of coffee, where I missed a flight to meet a famous neuroscientist. Folks should check out Nick Norwitz. Ended up in an ambulance with a heart rate in the toilet. Was even subjected to the embarrassment of my girlfriend's colleague seeing my naked white butt. But like the buzz from a good cup of coffee, that's gonna be a story that's delayed gratification. So wake up your brain, take a sip of caffeine, because I'm gonna teach you some wild and deeply practical science, and you're never gonna see a cup of coffee the same way again. Low HRV was a gong summoning the Grim Reaper. It's something you can actually measure today, understand, and improve. Heart rate variability isn't just some biomarker. It's a real-time window into how well your brain and body and heart are working together. Heart rate variability reflects something broader. Okay, real quick before we begin, if you want extra nuances, early access to this sort of deep dive content, you should check out our best-selling science newsletter, Stay Curious Metabolism. Find out why it's the fastest growing premium newsletter on the platform and why users exclaim it's the best investment they've ever made in their health. But plug over, let's dive in. This video is organized into what, why, How's and WTF. What is heart rate variability? Why does it matter? How do you measure it and how do you improve it? And then my WTF story. So let's start with what is heart rate variability? HRV. HRV is not your heart rate, the number of beats per minute, but the variability within those beats. Because your heart doesn't beat like a metronome. Tick, tick, tick. For example, if your heart beats at 60 beats per minute, one beat might take 1 second, the next 1.05 seconds, the next 0.92 seconds, and so on. Heart rate variability captures those microfluctuations in your heart's rhythm. And those microfluctuations are a direct signature of what your nervous system is doing with your heart, kind of written in doctor scratch like that on a heart tracing. And yeah, I'm going to show you how to read at least part of a heart tracing in a moment. Stay tuned for that. But back to HRV. Here's the physiology in plain English, or as plain English as I can make it for you. Your autonomic nervous system has two main branches: the sympathetic, fight or flight response, and the parasympathetic, the rest and digest response. The rest and digest response, the parasympathetic, is mediated by the vagus nerve, the 10th cranial nerve. But both the sympathetic and parasympathetic act on the heart's pacemaker system. The sympathetic system pushes your heart to beat faster and more steadily. The parasympathetic does the opposite. It slows things down, but more importantly, it does so rapidly and dynamically, like a good car brake, creating variability in each beat. So let's make it intuitive with an analogy. Think of your body as a car and your heart as the engine of that car, which it kind of is. The sympathetic nervous system is like pressing on the gas. You speed up and the ride becomes smooth and steady, low variability, although you're moving fast. So this is me in sympathetic drive. I'm moving quickly but very consistently. So heart rate variability is low. There's very little variability in my speed, no brake action. The parasympathetic system is like tapping the brake, small rapid adjustments that are kind of jerky and create subtle fluctuations in speed. Now I'm slowing down, my parasympathetic tone is up and I'm jerking all around. There's a lot more variability in my speed. That's parasympathetic drive. That's high heart rate variability. Now the analogy breaks down a bit here because while you're driving you don't want somebody tapping the brake and jerking you around, but in physiology that's exactly what we want. Those tiny adjustments reflect a brake system that's responsive, precise, and working. And obviously you don't want broken brakes. So seen this way, heart rate variability is a marker of vagal tone, parasympathetic strength, your ability to rest and digest or recover. But zooming out, heart rate variability reflects something even broader: your body's capacity for adaptive regulation, the ability to continually adjust our internal environment to meet external demands. This is fine. Or if you want to coin a phrase, neurocardiac agility. So to review, a high heart rate variability means a flexible, responsive system. A low heart rate variability means reduced parasympathetic vagal tone in a system that is less adaptable, more like flying down the highway of life with broken brakes. You don't want that. Hold on! So that's the what. Of heart rate variability. Now why? Why does heart rate variability matter? Okay, let's walk through a few landmark studies, starting with one published in the American Journal of Cardiology in 1987. In this study, researchers followed 808 patients who had already survived a heart attack. They put Holter monitors on them to track heart rate variability over the course of 11 days, looking specifically at variability in what's known as the RR intervals. Okay, pause. I'm throwing jargon at you. What the heck is an RR interval? Now is when we get to have a little fun. Because I could just say this is a measure of heart rate variability, but I want to explain to you what it is. And this is a great excuse to teach you, doctor, how to read an EKG. So the R wave on an EKG is that big spike. You've probably seen it, but you probably never asked what it was. It's actually really simple. When you're measuring heart activity, You put little leads all over and you can measure electrical signals coming out of the chest from the beating of your heart. The R wave, that big spike, represents the electrical impulse as it travels through the ventricles, the main pumping chambers of your heart. They depolarize, contract, and whoosh, blood flows out your heart and into your arteries. That is the R wave. So if you measure the time between those spikes,, and how much that time changes from beat to beat, you get heart rate variability, the microfluctuations in the beat. Simple enough, right? Good job, doctor, you can now read an EKG. Pat yourself on the back. Anyway, that's what the researchers measured, and then they grouped people based on their HRV. Remember, higher HRV is better, lower HRV tends to not be as healthy. So what did the researchers find? In the group with low HRV, survival plummeted. It was like low HRV was a gong summoning the Grim Reaper. I have come. These low HRV individuals had a 5.3-fold higher risk of death in the years that followed. And to be clear, it wasn't a short-term effect. These patients didn't just drop dead in the next few weeks. This was a longer-term predictor of mortality over years. That's really remarkable, don't you think? But hopefully you haven't had a heart attack. I haven't, and I'm hoping I never do. So you might be wondering, is this actually relevant to you and me? So let's look at another study where the goal was to examine whether low HRV predicts a first heart attack. To do this, researchers followed 2,501 individuals for an average of 3.5 years, and even after Adjustments for the usual suspects like age, sex, smoking, diabetes, and other risk factors, the signal held strong. Lower HRV was significantly associated with a higher risk of a cardiac event. Even a 1 standard deviation drop in HRV was associated with a 47% increased risk of a new cardiac event, like a first heart attack. So what we're seeing is profound. What we're seeing is low HRV predicts mortality after a heart attack, and low HRV also predicts who is more likely to have a first heart attack in the first place. That's a big deal. And yeah, it's a little unsettling, but as we'll see, it's also super empowering because now we get to the hows. What, why, and hows. How to measure and how to improve your heart rate variability. So let's start with how to measure your HRV. The good news is it's gotten a lot easier than since 1987. There are many wearables like Oura Ring or Whoop. I don't have any affiliations with either, but you can use wearables to track your HRV. You can also get a dedicated HRV tracker like Elite HRV. But when it comes to measuring HRV, you really have two main options for protocols. One, and this is what I do, is right after waking. This is while still laying in bed, before checking your phone, before caffeine, before food, before any real movement other than just grabbing your HRV device. This gives you a very controlled baseline with minimal external noise. It's a great baseline. You measure it day to day to day. The alternative is overnight measurements, which many wearables provide. The upside here is you get a much larger data sample— hours instead of minutes. The trade-off is it's a little bit less of an authentic baseline than a waking measurement since it can be influenced by sleep disturbances. So zooming out, the key is not to obsess over which one is better, resting in the morning or overnight. The key is consistency. Pick one that works for you, stick with it, and track your HRV over time across training cycles, stressful periods, travel, new medication, illnesses, seasons, all of it. Now, to the practical. The big question I know you've been waiting for: how do you improve your HRV? I'm going to go over 3 main ways. The first and best studied way to improve your HRV is regular aerobic exercise. I know it sounds boring, but it's the truth. This has been demonstrated across many systematic reviews and meta-analyses, including of randomized controlled trials. And honestly, the connection makes intuitive sense. It doesn't require a ton of exposition. The more important question, maybe the one more important to you, is what is the best type of exercise? For improving heart rate variability. Let's compare two types of aerobic training: zone 2 cardio and high-intensity interval training, or HIIT. Zone 2 cardio is performed usually at about 60-70% of your maximal heart rate. A quick heuristic for that is 220 minus your age, and that's your maximal heart rate in beats per minute, more or less. During zone 2 cardio, you can usually hold a conversation. Think of it as moderate cycling or light jogging for some people. This type of training consistently increases heart rate variability. So if you're starting from scratch, zone 2 is probably your most reliable way to increase your baseline HRV. HIIT is more complicated. HIIT is powerful, but it's a double-edged sword. It involves short bursts of very high-intensity efforts, sprints, intervals, all-out pushes. And in the short term, because of its intensity, you get a stress response and your HRV will drop. That's expected. But if dosed correctly, HIIT is an incredibly efficient way to improve your overall cardiovascular fitness, which can increase your HRV over time. The catch to HIIT is dosing. Too much and you can end up with a lower HRV because of poor recovery. But used strategically, it can be what really drives meaningful improvements in HRV once you're doing zone 2 cardio at a sufficient base. There's no correct ratio for how to balance these things, but to give you a practical starting point, Maybe make zone 2 cardio like 70% of your aerobic work and then have HIIT be the remaining 30%. As for what I do, I do HIIT 1 to 2 times per week. That's sufficient for me, keeping sessions short but really intense, usually 20 minutes twice per week. Then use your HRV to personalize your protocol. That's the whole point of tracking it. If you're measuring HRV daily and do a hard HIIT session and your HRV drops, that's not a problem per se. That's a signal. It's the signal your body needs recovery, so let your HRV come back up before your next hard session. Now, before we get to the next two ways to improve your HRV, let's discuss a common drug that lowers HRV. You know it, you love it— caffeine. But caffeine isn't dangerous, and here's why. Over the short term, caffeine suppresses HRV by shifting your nervous system towards that sympathetic fight or flight state, pushing the gas on your nervous system. Lower HRV. Smooth but fast ride. That said, it's a temporary controlled stressor, similar to exercise. And both exercise and caffeine can paradoxically improve HRV when used and dosed appropriately. In general, and here's some good news, up to 4 cups of coffee per day is generally either neutral or beneficial for cardiovascular and overall health, especially when consumed in the morning. Just not before you take your morning HRV test. Now back to the other two ways to improve HRV. One nutrient shown in multiple randomized controlled trials to boost HRV is omega-3s, specifically the long-chain omega-3s EPA and DHA. They do so via many mechanisms: improving membrane fluidity by being incorporated directly into cell membranes of neurons and heart cells. They also reduce chronic inflammation, which can otherwise suppress HRV. And finally, EPA and DHA act through the central nervous system, through the the brain by accumulating in key brain regions like the brainstem, which can help to improve vagal tone. As for dosing omega-3s, extrapolating from the randomized controlled trials, between 3,000 and 4,000 milligrams per day of combined EPA and DHA represents the clinical-grade range. You can reach this with supplements, but also with food. In fact, just 3 cans of sardines per day will land you in that 3,000 to 4,000 milligram range. And perhaps not coincidentally, My HRV was never higher than when I did my sardine diet experiment where I was consuming a minimum of 6,000mg of EPA and DHA daily. And if you're into supplementation, my go-to brand is Eccentrate Omega Max for omega-3s. It includes a special form of omega-3 called lysophosphatidylcholine DHA, which gets into the brain and brainstem, relevant to HRV, more efficiently than a standard fish oil. The total dose of omega-3 is lower, than some other products, but this is because this form is harder to concentrate, and in many cases, including beyond HRV, brain delivery matters more than raw dose, which is why I like this supplement. I still consume this on the back of a fish-rich diet, but this is what I personally use and stand by. Now, as a quick caveat, you never need to buy anything from me, but I know a lot of you ask, so that's why I share this, my answer for what I use. And if you want to try it, you can use my discount code STAYCURIOUS for 20% off. And before we get back to our main video, I do want to be transparent. I earn commissions on this product. It is sponsoring this video, but that money goes right back into this channel. Things like editing fees— editing fees are not cheap— and even paying open access fees for research papers and funding experiments that I will share here, including one I have coming that cost me a few thousand dollars, actually upward of $10,000, and 7 years of my life. So stay tuned for that. But now back to the video. I just want you to know that I invest what I make from you back into educating you. Okay. I got that off my chest. But the third thing that improves heart rate variability is heart rate variability biofeedback. It's a protocol to improve your heart rate variability that is exactly what it sounds like. Structured breathing while monitoring your heart rate variability in real time. To train your nervous system. The core principle here is something called a resonance frequency. This is the breathing rate at which your heart, lungs, and blood pressure all synchronize to produce the strongest and most efficient HRV signal. You can think of it like pushing someone on a swing. Each time you push perfectly, each swing builds on the last and the amplitude builds, in this case, the heart rate variability builds. Here, the oscillations are your heart rate variability. In other words, it's the sweet spot where your nervous system responds most powerfully to your breath. For most people, this sits in the 5 to 6 breaths per minute range, or 10 seconds per full breath. So a 5-second inhale and a 5-second exhale. I don't want to actually do that exercise for you because 10 seconds is actually a long time in YouTube video time. That is a great place to start. And research shows that heart rate variability biofeedback can increase HRV, improve stress resilience, and even have a positive effect on anxiety, depression, anger, and athletic performance. So here's how you do it in a nutshell: find your breathing rhythm, start at 5 to 6 breaths per minute or 1 breath for every 10 seconds, and then adjust slightly based on what feels most natural and sustainable for you. Then practice this for 10 to 20 minutes 1 to 2 times per day. If you're using a device, you can actually watch your heart rate variability waveform in real time, and you're looking for smooth, large oscillations. During the actual session, you might see your HRV typically increase as you drift into the parasympathetic state. And over time, usually 4 to 8 weeks of this training with biofeedback, you may even see modest increases in your baseline HRV with morning or overnight testing. But now, zooming out, let's look at the big picture. Heart rate variability isn't just some biomarker for physiologists or elite athletes. It's a real-time window into how well your brain and body and heart are working together, how efficiently you can adapt and recover and respond to stress. It predicts death in some people. It predicts first heart attacks in others. But more importantly, it's something you can actually measure today, understand, and improve. Now, the video could end there. But I really want to pay off the opening hook and tell you about the craziest coffee and Starbucks story you have ever heard, guaranteed. This was a few years ago and I was headed to Los Angeles for a business trip, a few meetings, including a very generous offer to meet with Andrew Huberman. The date was precisely July 20th, 2024. That was the day, if you remember, of a massive airline tech meltdown Thousands of flights were canceled. On Delta alone, something like 1.3 million passengers were affected. But I still wanted to make my trip to LA, so I scrambled, rebooked, and ended up stuck in Boston Logan Airport for several hours. I figured I'd be productive. I was a med student, I had a lot of work, so I went to a Starbucks to get a drink and do that work. I ordered a mint tea. I also lied at the beginning because it tea, not a coffee. Sorry about that. Anyway, that was my first mistake. Now, I don't know if you've ever been to Starbucks recently, but they serve their tea hot, and this one was not hot, it was volcanic, and it was filled to the absolute brim and even double cupped. It was burning my hand, so I set it down and tried to pop the lid off to pour a little out and put in some cool water. That was mistake number 2, because as I lifted the lid, boiling water spilled onto my hand, and I reflexively dropped the entire cup over myself. And instantly I knew it was bad. And then it got worse as the burn started to set in. So I ripped my shirt off in the middle of Boston Logan Airport. But it gets worse because the liquid had soaked through all my clothes, including down my leg into sensitive territory. So now I was already shirtless in the middle of Boston Logan Airport with kids with little balloons staring at me like, what the heck's going on? I had a hard decision to make: pants on or pants off? Because like I said, there were children around, I made the noble sacrifice of sprinting to the bathroom before taking my pants off and jumping in the sink. Unfortunately, the delay was enough. The burns had set, especially around my hip and butt. The blisters started to form pretty quickly, and long story short, I ended up with 20% body surface area burns, mostly second degree. I'm a Starbucks mint tea. Now thankfully, except for a little scar on my right butt cheek, I've made a full recovery. No, I'm not going to show you that scar. But what ended up happening was EMTs had to come pick me up and bring me in an ambulance to the MGH emergency department, where who cared for me other than my girlfriend's colleague at Massachusetts General Hospital, who is a surgical resident. So yeah, my girlfriend's colleague got to look at my naked, white, blistering butt. That was awkward. But here's where things get physiologically interesting. That level of burn triggered a systemic response in my body. My body essentially started going into shock. My blood pressure, which usually sits around 100 millimeters of mercury systolic, dropped, then shot up to 200 and bounced around a bunch. Now, if you know about basic physiology, you'd expect my heart rate to go up with this stress response. But instead, I had a weird, bizarre reflexive bradycardia. My nervous system was all over the place. Blood pressure up and down, heart rate fluctuating wildly, at times dropping into the 20s and 30s. And if anybody does have an explanation for the exact series of biological events that went on in my body, please feel free to leave it in the comments. But one interpretation, especially through the lens of HRV and related topics, is that my paradoxical drop in heart rate may reflect a very strong vagal parasympathetic tone. Maybe I'm stretching here, but it would be consistent with a high heart rate variability. So yeah, I had to cancel my trip, which was a bummer. And to his credit, Andrew Huberman was incredibly kind and concerned when I explained why I had to cancel. But on the bright side, this might be the most painful confirmation ever that I've got pretty solid vagal tone. So I guess there's a silver lining in there. Maybe I'm stretching a bit, but hopefully this story was entertaining and one you'll remember. And hopefully my heart will keep me going for many years to come, provided I don't take myself out first with a tea or coffee. Stay curious. I hope you learned something and hope you had fun.