The world, Kristen Holmes told me, quietly splits into two kinds of people: those who can steer their own autonomic nervous system, and those who get steered by it. The first group does not just carry a competitive edge. In her words, they carry a survival advantage.
Here is the part almost nobody connects. A growing body of work suggests that much of what we file under aging, the cellular wear, the mitochondrial slowdown, the low-grade inflammation that creeps in over the years, may be downstream symptoms of a nervous system stuck in the wrong gear. Kristen pointed me to a 2025 paper making exactly this case: that the classic hallmarks of aging read, in part, like the signature of a dysregulated autonomic nervous system.
I sat down with Kristen Holmes, Global Head of Human Performance and Principal Scientist at Whoop, and the author of a new book, Aligned, that puts the autonomic nervous system at the center of health rather than out at the margins. She has access to one of the largest datasets on human physiology on the planet, and she wrote a book that is less about squeezing out performance and more about wholeness. Here is what she taught me about the master variable most of us never think to train.
Two branches, one modern problem
Your autonomic nervous system runs on two branches. The sympathetic branch is the accelerator: activation, hard-charging, go. The parasympathetic branch is the brake: rest, digest, restore. Health is not living in one or the other. It is moving fluidly between them, gas when you need it, brake when you do not. The modern problem is that a lot of us get stuck with a foot lightly on the accelerator all day long.
Trauma is the extreme version of that, a system locked into sympathetic overactivation, and Kristen noted that chronic sympathetic dominance is itself a hallmark of aging and disease. To make the point, she pointed to the work of Dr. Eugene Lipov, whose stellate ganglion block, a targeted nerve block, has produced striking and durable relief in special operators with treatment-resistant PTSD, essentially by releasing the stuck accelerator so the brake can take over. Most of us are nowhere near that extreme. But most of us are also not spending nearly enough time on the brake.
HRV is the window in
The way we measure this balance is heart rate variability, the tiny, constantly shifting gaps between your heartbeats. This one is counterintuitive: more variability is better. A heart that ticks like a metronome is a warning sign, not a healthy one. Kristen described HRV as the best available proxy for how balanced your nervous system is, a point a 2018 meta-analysis helped establish. Genetics load the dice, research puts the heritability of HRV somewhere between roughly 47 and 65 percent, but her entire message is how much of it is trainable. Wherever you start on that range, the levers still work.
Trust and fear are physiology, not just feelings
Kristen sorts emotions into two families. There are emotions of trust, joy, kindness, peace, harmony, and emotions of fear, insecurity, jealousy, shame, guilt. Fear emotions are not the enemy. They pass through all of us. The problem is getting stuck in them, because living from fear means living in the sympathetic branch, and that, she argues, quietly accelerates the pace of your aging.
The tool she offers is almost embarrassingly simple: affect labeling. Name the emotion, get curious about it, and you loosen its grip enough to reframe the moment. Then her data delivers the punchline. Her team published work in the journal Emotion showing that whether you appraise your day as a threat or as a challenge barely changes your heart profile during the day, but it shows up hard at night. Appraise the day as threatening and you do not drop into the deep, restorative stages of sleep; your HRV stays suppressed and your resting heart rate stays elevated. How you think while the sun is up gets cashed out in how you recover after it goes down.
Got amplitude?
Here is the big lever, and Kristen wants it on a t-shirt: circadian amplitude. Picture your physiology as a wave rolling across twenty-four hours. Amplitude is the size of the swing between your daytime state and your nighttime state. A big, clean swing is a marker of health. A flat line is what you tend to see in the sick, the unfit, and the very old.
You build amplitude through contrast that is aligned with the natural light-dark cycle: bright, active days and dark, quiet nights. Be genuinely busy when the sun is up and genuinely still when it is down. Eat in daylight and stop eating at night. Underneath all of it sits the humble behavior her data keeps flagging as most important, and the one everyone hates to hear: sleep-wake consistency, going to sleep and waking at similar times. She has a paper in review on roughly 5,000 peri- and postmenopausal women suggesting that higher amplitude tracks with fewer and milder symptoms. The effects are modest and the work is preliminary, but the direction is telling. And the reset is fast: studies show that a couple of weeks of camping with no artificial light can pull a scrambled clock back into line.
Light and movement are not optional
Two inputs feed your amplitude more than any gadget. The first is light. We spend something like 93 percent of our lives indoors, and Kristen puts the minimum effective daily dose of natural light somewhere near 90 minutes, which almost no one is getting. Get outside, early and often, skip the sunglasses when it is safe to, and bank light the way you would bank sleep.
The second is movement, and not the way most of us do it. The problem is not only the workout you skipped; it is the eleven sedentary hours wrapped around the one you did. Humans used to move something like seven to ten miles a day. Kristen’s answer is all-day movement, what she calls exercise snacks: a hundred kettlebell swings between meetings, a walk as the sun sets, a jump rope next to a standing desk. In her data, which spans hundreds of thousands of people, the ones who move more sleep better, and the fitter ones sleep more efficiently, closer to seven hours and twenty minutes than to eight and a half.
Train for contrast, not just for muscle
This is the reframe that reorganized my own thinking. I tend to evaluate training in one dimension: is it building strength and muscle? Kristen made me add a second: what is it doing to your amplitude and your recovery? Intensity matters, but not every day. A few genuinely hard sessions a week create the sympathetic spike you want. Then zone one and zone two work, the long walk, the easy jog, does something most people miss entirely: it engages the parasympathetic brake and builds vagal tone. She almost refuses to file it under exercise at all.
And here is the warning aimed squarely at my audience. The people overtraining are rarely the elite athletes, who program recovery to the millisecond and cannot afford to overreach. It is the driven recreational athlete who believes more is always better. Your HRV will tell you that you have crossed the line before your body forces the issue. That is the athlete’s trap, the hard-won ability to override your own signals and push through, and it is genuinely dangerous. The fittest among us are often the most disconnected from what their body is actually saying.
A few things to carry with you
- Anchor your wake time. Sleep-wake consistency is the foundation of a robust nervous system, and the single lever almost everyone underrates.
- Build contrast. Bright, active, well-fed days and dark, still, food-free nights. The bigger and cleaner the daily swing, the healthier the signal.
- Get outside. Aim for meaningful natural light every day, banked across the whole day the way you would bank sleep.
- Move all day, not just once. Sprinkle in exercise snacks so a single workout is not marooned in a sea of sitting.
- Train for recovery, too. Go genuinely hard a few days, keep the rest in zone one or two, and let your HRV, not your ego, tell you when to back off. Skip high-intensity work in the roughly four hours before bed, which her team’s data links to worse sleep.
- Weight your eating toward daylight. A roughly 12-hour window, with most of your calories earlier in the day, gives your nervous system the nightly rest it needs to recover.
Kristen closed on something I have believed for years: behavior precedes the thought. You cannot think your way into a better nervous system. You do the thing, the light, the movement, the consistent sleep, the honest hard training, and the feeling follows. None of this is medical advice, and I am not your doctor. It is education, so you can look at your own days and nights with a clearer eye and build the contrast your physiology is quietly asking for. Your muscle is built in the active phase and repaired in the quiet one, so defend both halves of that rhythm. Until next time, stay forever strong.
