Your nervous system runs on a budget you can learn to read
Applied Physiology #1 - A hard conversation and a hard interval get charged to the same account, and hardly anyone plans a week as though that were true.
Applied physiology series, part one. This one reads recovery as a single account that everything in a week draws on, before part two turns to the withdrawal that pays back most, which is strength training after 40.
Take an ordinary Tuesday out of last summer. One of mine, in Lisbon.
Nine in the morning, a call about money. The useful part of it lasted maybe four minutes and I carried the other fifty around for the rest of the day. Then the middle of the day, which is where the small decisions live, six or seven of them, none of them big on its own, every one with a clock attached. My sister and I were building the company and there was nobody to hand any of it to. By six the inbox had gone quiet, which felt like permission, and the session was on the plan anyway. Ninety minutes of threshold work. The kind that’s supposed to hurt in a boring, predictable way.
It didn’t hurt in a boring, predictable way. Someone had moved the intervals up a gear while I wasn’t looking, or that’s how it felt, and I finished it mildly annoyed at my own legs.
Then a short night, because the day had run long and the alarm doesn’t negotiate.
Nothing in that Tuesday was a mistake. The call had to happen, the decisions were mine to make, and the session had been written down weeks earlier by a version of me who thought it was a sensible idea at the time. What that day actually was is four withdrawals from one account. Your body doesn’t keep separate books for work and for training, and I spent about a decade assuming it did.
The physiology behind that is less complicated than it sounds. The system that runs your heart rate and your digestion answers to a single pool of capacity, a difficult conversation draws that pool down in much the way a difficult interval does, and once you can see the balance, most of the arguments you have with yourself about discipline stop being worth having.
This is an educational and strategic perspective, not personal medical advice. The views are the author’s own and not statements by Atlas Cove Lda.
The body keeps one set of books
The autonomic nervous system runs everything you never think about: the heartbeat, the gut, the immune traffic, the repair that gets going once you stop moving. It has two branches and they pull in opposite directions on purpose. The sympathetic branch puts you into output. Heart rate climbs, blood gets pushed toward the muscle that is about to work, and the hormones that make hard effort possible go out into the blood. The parasympathetic branch does the other half of the job, so the heart slows, the gut gets its blood supply back, tissue repairs itself and glycogen refills.
Here’s the part that matters if you train seriously and also work for a living. Both branches answer to overlapping central stress-response networks, so a threshold interval, a difficult negotiation, a short night and a low-grade cold do not get booked to four separate ledgers (Chrousos, 2009). They pull on one shared reserve. The system never asks where a demand came from, only how big it was and how much came back afterwards.
That’s where the intuitive model of stress falls over. Most people I know manage their training load with real care, down to the week and the session, and then treat the other hundred and ten hours as though those were free. The deadline and the broken night are withdrawals in exactly the sense the interval was. I did this for years and I’ll come back to how obvious it looks now.
The bill arrives as nothing dramatic
In short bursts none of this is a problem, and that’s worth saying before anything else. A sympathetic surge in response to a hard effort or an acute mess is precisely what the machinery exists for, and given a decent night afterwards it resets at no cost. The trouble starts when the demands come often and the recovery between them gets thin.
The idea that describes the long-run cost of that imbalance is allostatic load, which came out of Bruce McEwen’s work on the physiology of stress. It names the wear that the regulating systems take on when they get called repeatedly without enough recovery between calls (McEwen, 1998). Wear is the right word for it. Nothing breaks.
What it looks like in an ordinary week is undramatic to the point of being easy to miss. A resting heart rate a few beats above your own normal. Sleep that no longer puts you back where it used to put you. Body composition that won’t move on a routine that hasn’t changed, and attention that thins out around three in the afternoon. None of that is a verdict on your character and none of it is a diagnosis of anything at all. It’s a system spending less because less is what it has this month, which is a sensible thing for a system to do.
The daily version reads like that Tuesday, repeated. The morning meeting sets off an early hormonal response, the decisions under time pressure keep it up, and the evening session adds a physical demand on top of a cognitive one that never cleared. Then the short night fails to return what the day took out. Each of those is manageable on its own. They arrive in that order for weeks at a time, and then they add up to a load that no single night clears, so the shortfall carries into the week after.
I’ve had a long stretch with this account overdrawn, and I’ve written about that elsewhere, so one line is enough here.
The side of the account almost nobody trains
The restoring side isn’t fixed, which is the encouraging half of all this. It answers to a stimulus much the way your aerobic engine does, so you can build it deliberately instead of waiting around to feel better.
Easy aerobic work is associated with better cardiac parasympathetic regulation (Buchheit and Gindre, 2006). The slow hours build the branch that puts things back, and most people are already doing those hours for a completely different reason. Slow breathing, somewhere near six breaths a minute, moves autonomic balance toward the parasympathetic side within minutes, and a review of work in healthy adults suggests the benefit accumulates with regular practice (Russo, Santarelli and O’Rourke, 2017). Deep sleep is when parasympathetic traffic runs highest. Which is why guarding your bedtime and your evening light pays back more than almost anything you could swallow instead.
Now the limits, because I don’t want that read as stronger than it is. Buchheit and Gindre is an association in people who already train, and the breathing work is a review of small studies in healthy adults, so what I’d claim out of both is the direction. The dose is guesswork. There’s also no trial I know of that takes recovery capacity as its endpoint and runs for a year. I’d like to see one.
Here’s the accidental part, and it took me embarrassingly long to notice. Most of my week was easy aerobic work, done entirely for the engine. Or so I thought. Those same hours were the only thing I was doing for the other side of the account, and I was doing it by accident, which is roughly the level of intent I brought to recovery back then.
The pattern most people fall into is to train the spending side to exhaustion, through hard sessions and sheer volume, to leave the restoring side completely untrained, and then to read the slower recovery that follows as an inconvenience rather than as information. Building the restoring side is one of the few moves available that makes the budget bigger, instead of rationing what’s left of it.
Two kinds of number, and only one answers today’s question
There’s a difference between measuring what your body has produced and measuring what it can currently carry, and the gap between those two matters more than it first looks.
Outcome measures describe where you’ve been over months. Cholesterol, VO2 max, body composition, whatever your last blood panel said. All worth having, and I get mine done. They tell you very little about what you can absorb today. State measures get closer to that question: your morning heart rate, the trend in your heart rate variability, what the last two nights were actually like, how heavy the first ten minutes of easy work feel. Heart rate variability has been studied specifically as an index of how strongly the brain’s regulating regions are shaping autonomic output, which is part of why it tracks with stress and health rather than with fitness considered on its own (Thayer et al., 2012).
The version you can run this week is small. Before the session you’ve already decided on, take four readings together rather than one at a time: your resting heart rate this morning held against your own normal and nobody else’s, whatever recovery trend the watch gives you read across a week instead of as today’s verdict, what the last two nights were genuinely like including the one you’d rather round up, and how heavy the first ten minutes of easy work feel. That last one needs no device at all. It’s the one I’ve come to trust most.
One of the four pointing down is noise. Three or four pointing the same way is a week you won’t win an argument with, and the useful move is to put something back rather than take more out, whatever the plan says. When they’re all fine, spend freely. That’s the whole lens.
Precision is not what any of this offers. A single morning number tells you close to nothing, consumer devices disagree with each other and sometimes with themselves a day later, and the signal lives in the direction across a week. Read the state, then decide. Collecting a good number is a different hobby, and a surprisingly absorbing one.
My spreadsheet had one column in it
I ran a version of that for years, carefully, on exactly one side of the account. Managing a training load is genuinely something I know how to do, and I’ve taken that part seriously for a long time. Every week of the last build was planned properly, the hours and the split between easy and hard. I knew what would come off the plan when a week went wrong, before the week went wrong.
Then look at what else was in those months. We were building a company in a country whose paperwork I was still learning, with money decisions in most weeks, and the occasional four in the morning where I solved something in my head and then had to solve it again properly at nine.
None of that appeared anywhere in the plan. The spreadsheet had a column for training hours and no column for anything else, and I treated everything outside it as free. I’d have laughed at anyone modelling a system that way at work.
The physiology says the two withdrawals are the same withdrawal. The hypothalamic-pituitary-adrenal axis, which runs the hormonal arm of the stress response, doesn’t keep one programme for a hard session and another for a hard week (Chrousos, 2009). Same pathway. Same reserve.
There’s evidence for the interference beyond the mechanism, and I want to be straight about what kind of evidence it is. A systematic review of stress and exercise reported that higher psychological stress goes with poorer recovery, smaller training adaptations and more injuries at a comparable training load (Stults-Kolehmainen and Sinha, 2014). Those things travel together, which is all a design of that kind can establish, and it deserves saying plainly rather than slipping past. Nobody randomises a person into a stressful year. Anyone under strain also sleeps worse and eats differently, so isolating the strain itself is not something these designs can do. What it supports is narrower and still useful: a plan that looks perfectly reasonable on paper can be the thing that tips the total over, because the plan never knew about the rest of the week.
While I’m here, an opinion, flat. Every few months there’s a fresh exposure to be frightened of, the pan, the receipt paper, the bottle, whatever’s in the shampoo. Most of it is noise. The load I’d actually put money on, for most of the people I know, is the strain that never lets up, and that one never makes it into a headline, because there’s no product to put next to it. A pan you can replace on a Saturday morning.
Where this leads
A budget with a balance in it is a model, and every model has an edge where it stops being true, so here is this one’s. Nothing in you is totalling the week. There’s no organ computing a balance, and I find tidy arithmetic reassuring, which is a fact about me rather than about the body. What the model gets right is the pooling, one reserve with many claims on it. What it gets wrong is precision, and once you start pricing your Tuesdays in points you’ve traded one delusion for a better dressed one.
A hard week isn’t damage, either. The system is built for load and gets better at carrying load by being loaded, and what it needs is for the other half of the cycle to actually happen. That half is the one that gets negotiated away first.
There’s a bigger claim I hold and can’t support at the strength I’d like. A meaningful share of what gets treated as purely a head problem is, I think, sitting on top of a body that’s been under-recovered for a long time. That’s a belief. The literature I can point at is associations, and I’d rather say the thing with its label attached than dress it up as settled.
State before load is the order we built the Atlas Cove week around. What a guest can currently carry gets established before anybody decides what to ask of them, and it gets read again at the end to see which way it moved.
Go back to that Tuesday. What I told myself afterwards was that I’d gone soft, which is the nearest explanation to hand and costs nothing to reach for. The legs had carried a call about money and six or seven decisions with clocks attached before anyone asked them for an interval, and the night that was meant to pay for all of it got cut short. Add those up. The evening explains itself. Call it a character flaw instead and there’s nothing left to do with it, which is usually the sign that you’ve picked the wrong explanation.
This is an educational and strategic perspective, not personal medical advice. The views are the author’s own and not statements by Atlas Cove Lda.
Sources
McEwen, B. S. (1998). Protective and damaging effects of stress mediators. New England Journal of Medicine, 338(3), 171-179. DOI: 10.1056/NEJM199801153380307
Chrousos, G. P. (2009). Stress and disorders of the stress system. Nature Reviews Endocrinology, 5(7), 374-381. DOI: 10.1038/nrendo.2009.106
Stults-Kolehmainen, M. A., & Sinha, R. (2014). The effects of stress on physical activity and exercise. Sports Medicine, 44(1), 81-121. DOI: 10.1007/s40279-013-0090-5
Buchheit, M., & Gindre, C. (2006). Cardiac parasympathetic regulation: respective associations with cardiorespiratory fitness and training load. American Journal of Physiology-Heart and Circulatory Physiology, 291(1), H451-H458. DOI: 10.1152/ajpheart.00008.2006
Russo, M. A., Santarelli, D. M., & O’Rourke, D. (2017). The physiological effects of slow breathing in the healthy human. Breathe, 13(4), 298-309. DOI: 10.1183/20734735.009817
Thayer, J. F., Åhs, F., Fredrikson, M., Sollers, J. J., & Wager, T. D. (2012). A meta-analysis of heart rate variability and neuroimaging studies: implications for heart rate variability as a marker of stress and health. Neuroscience & Biobehavioral Reviews, 36(2), 747-756. DOI: 10.1016/j.neubiorev.2011.11.009