What accumulates when stress repeats

Nothing in particular went wrong last year. That is what makes it difficult to describe to anyone. A parent needed more phone calls than before. Work reorganised twice. Sleep was fine on paper and never quite finished the job. By spring you were carrying something, and there was no single event to hang it on.

The previous articles followed one stress response from beginning to end: the off switch, the three-organ relay, and what cortisol does while it is circulating. This one takes the fourth question in the series. When the loop runs again and again over months, with too little room in between, something accumulates.

Stability through change

Your body does not hold its values fixed. Blood pressure climbs to meet the morning and falls again at night. Glucose is released and cleared. Inflammatory signalling rises to deal with an injury and then resolves. The physiologist Peter Sterling called this allostasis: stability achieved through change rather than through holding still.

To understand it, imagine riding a bicycle. You do not stay balanced by keeping your body completely stiff and frozen; you constantly lean, shift your weight, and adjust your handlebars to stay upright when you hit bumps or windy spots. Your body does the same. Internal levels like blood pressure, blood sugar and inflammation do not stay locked at one single, fixed number. This flexibility is the whole point of your body’s design.

Allostatic load is the cost of running that flexibility continuously. When the next demand keeps arriving before the previous one has settled, the settings drift. Inflammatory markers do not fully resolve between exposures. Glucose regulation loosens slightly. Resting cardiovascular tone shifts.

Imagine carrying a backpack. Carrying a lunchbox in your bag for an hour is easy. But if people keep dropping heavy rocks into your backpack without giving you a chance to take a break and unpack, your shoulders get tired, your posture changes, and you start dragging your feet. Allostatic load is the biological wear and tear of running that constant flexibility without enough time to recover. On any single day, none of these small changes feel dramatic, which is precisely why allostatic load can build up unnoticed for years.

The evidence that this matters

Researchers gathered every study they could find on this question and combined the results into one bigger picture. People whose stress had built up the most were 22 percent more likely to die of any cause, and 31 percent more likely to die specifically from heart and blood vessel disease, compared with people whose stress load had not built up as much (Parker et al., 2022, American Journal of Preventive Medicine). All seventeen studies pointed the same way. A separate review combining 267 studies found the same basic pattern across many different physical and mental health problems (Guidi et al., 2021).

Things worth knowing about those numbers: the studies did not all measure stress load in exactly the same way. And none of this proves that stress load directly causes death. Researchers followed groups of people over time and watched what happened, rather than testing it directly in an experiment. That kind of study can show two things are linked. It cannot, by itself, prove one caused the other. What holds up is the pattern itself, showing up again and again across very different groups of people and different studies.

The finding that changes what you look at

For thirty years, one big criticism of this idea was that different researchers measured it in different ways, which is part of why the studies did not agree closely with each other. In 2023, one group tackled that problem head-on. They combined the actual data from 67,126 people, aged 40 to 111, across thirteen long-term studies, looking at 40 different body measurements spanning twelve body systems.

They wanted to find the smallest set of measurements that could still predict health outcomes just as well. They found five, and this small group predicted death from any cause as well as, or better than, the bigger, more complicated versions: a marker of inflammation called C-reactive protein, resting heart rate, “good” HDL cholesterol, the ratio of your waist size to your height, and a blood sugar marker called HbA1c.

Cortisol is not one of them. That is worth pausing on, because it changes how you should think about the whole subject. Cortisol is the messenger that sets off the stress response. What actually builds up over years is not the messenger itself. It is what that messenger keeps doing to the rest of the body: inflammation that never quite settles down, blood vessels that stay slightly more tense than they should, fat handling that shifts, blood sugar control that gets looser.

How far the recovery story can be pushed

The cortisol spike itself is a normal, healthy response, and what actually matters is whether your body comes back down to normal afterward, cleanly. One study tested this directly. Researchers put 130 healthy people through the same stressful task, then measured two separate things: how much each person’s cortisol rose and how quickly it came back down afterward. Recovery did not turn out to be a better sign of long-term stress than the initial rise was. A study the following year added a further complication, finding no real overall link between daily cortisol patterns and how much someone’s cortisol spiked under stress, and raising doubts about whether the usual ways of measuring “recovery” are even reliable.

The idea that recovery matters more than the spike is reasonable, and scientists are actively studying it, but it has not been proven yet.

What this leaves you with

You cannot measure your own build-up of stress with a cortisol test or a wearable device. A saliva cortisol test only catches you at one moment on a curve that naturally rises and falls throughout the day. A smartwatch stress score is built almost entirely from heart rate variability, which reflects the balance between the two branches of the autonomic nervous system: the fast system that governs moment-to-moment arousal.

More useful: the five measurements that do carry real information are ordinary and easy to get. C-reactive protein, HbA1c and HDL cholesterol all show up on a standard blood test. Resting heart rate and the ratio of your waist size to your height, you can measure yourself at home. What matters is watching how those numbers move over several years, not any single result.

If you take one thing from this

The next time you get routine blood work done, ask to see the older results next to the new ones. You are not looking for one abnormal number. You are looking for a direction: whether inflammation, blood sugar control and fat levels have been quietly drifting in one direction over several years. That slow drift is what built-up stress looks like once it becomes visible, and it becomes visible long before anyone would call it a disease.