Heart Rate Variability (And The Coffee that Almost Killed Me)
In a Nutshell
Heart rate variability (HRV) measures micro-fluctuations between heartbeats and serves as a direct marker of vagal tone and the body's ability to adapt to stress. Low HRV predicts a 5.3-fold increase in mortality after heart attack and a 47% higher risk of first cardiac events. To raise HRV, prioritize zone 2 cardio (70% of aerobic work), 1–2 short HIIT sessions weekly, 3–4 g/day combined EPA+DHA, and 10–20 minutes of daily resonance breathing at ~6 breaths per minute.
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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 fivefold 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.
The story of nearly killing myself with a cup of coffee, missing a flight to meet a famous neuroscientist, ending up in an ambulance with a heart rate in the toilet, and being subjected to the embarrassment of my girlfriend's colleague seeing my naked white butt will be told later as delayed gratification.
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 even broader.
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. 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 micro fluctuations in your heart's rhythm. And those micro fluctuations are direct signature of what your nervous system is doing with your heart.
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. 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.
Think of your body as a car and your heart as the engine of that car. 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. 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.
Heart rate variability is a marker of vagal tone, parasympathetic strength, your ability to rest and digest or recover. Heart rate variability reflects 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.
A high heart rate variability means a flexible, responsive system. A low heart rate variability means reduced parasympathetic vagal tone and a system that is less adaptable, more like flying down the highway of life with broken brakes.
A landmark study published in the American Journal of Cardiology in 1987 followed 808 patients who had already survived a heart attack. Researchers put Holter monitors on them to track heart rate variability over the course of 11 days, looking specifically at variability in the RR intervals. In the group with low HRV, survival plummeted. These low HRV individuals had a 5.3-fold higher risk of death in the years that followed. This was a longer-term predictor of mortality over years.
Another study examined whether low HRV predicts a first heart attack. Researchers followed 2,501 individuals for an average of 3.5 years. Even after adjustments for age, sex, smoking, diabetes, and other risk factors, lower HRV was significantly associated with a higher risk of a cardiac event. Even a one standard deviation drop in HRV was associated with a 47% increased risk of a new cardiac event, like a first heart attack.
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.
The R wave on an EKG is that big spike. 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. If you measure the time between those spikes, and how much that time changes from beat to beat, you get heart rate variability, the micro fluctuations in the beat.
There are many wearables, like Oura Ring or Whoop, to track your HRV. You can also get a dedicated tracker, like Elite HRV.
When it comes to measuring HRV, there are two main options for protocols. One is right after waking, 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. 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.
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.
The first and best-funded way to improve your HRV is regular aerobic exercise. This has been demonstrated across many systematic reviews and meta-analyses, including of randomized controlled trials.
Zone two cardio is performed usually at about 60 to 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 two 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 two is probably your most reliable way to increase your baseline HRV.
HIIT is powerful, but it's a double-edged sword. It involves short bursts of very high-intensity efforts, sprints, intervals, all-out pushes. 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 two cardio at a sufficient base.
A practical starting point is to make zone two cardio like 70% of your aerobic work and then have HIIT be the remaining 30%. HIIT one to two times per week is sufficient, keeping sessions short but really intense. Usually 20 minutes twice per week. Use your HRV to personalize the protocol. 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, you let your HRV come back up before your next hard session.
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, up to four 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.
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 brain, by accumulating in key brain regions like the brain stem, which can help to improve vagal tone.
As for dosing omega-3s, extrapolated from the randomized controlled trials, between 3,000 and 4,000 mg 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 three cans of sardines per day will land you in that 3,000 to 4,000 mg range. My HRV was never higher than when I did my sardine diet experiment, where I was consuming a minimum of 6,000 mg of EPA and DHA daily.
Xcentrate Omega Max for omega-3s includes a special form of omega-3 called lyso-phosphatidylcholine DHA, which gets into the brain and brain stem 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.
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 breath per minutes range, or 10 seconds per full breath. So, a 5-second inhale and a 5-second exhale. 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.
Find your breathing rhythm. Start at 5 to 6 breaths per minute or one 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 one to two 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.
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 in response to stress. It predicts death in some people. It predicts first heart attacks in other. But more importantly, it's something you can actually measure today, understand, and improve.
This is 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 went to a Starbucks to get a drink and do that work. I ordered a mint tea. 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. As I lifted the lid, boiling water spilled onto my hand, and I reflexively dropped the entire cup over myself.
Instantly, I knew it was bad. And then it got worse as the burns 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 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, from a Starbucks mint tea.
Thankfully, except for a little scar on my right buttock, I've made a full recovery. 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.
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 mm of mercury systolic, dropped, then shot up to 200, and bounced around a bunch. 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 shot up and down, heart rate fluctuating wildly, at times dropping into the 20s and 30s.
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.
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