There is a kind of tension that arrives the evening before something difficult. Not when the difficult thing starts, in the hours before it, while nothing is actually happening yet. The Sunday afternoon that turns, for no event you can point to. The night before the appointment, the confrontation, the morning you have been dreading all week. The body seems to have read the calendar before the day has done anything to earn the dread.

For a long time I treated this as a quirk of temperament: the way some of us are simply more prone to running ahead of ourselves. The research suggests something more precise, and to me more reassuring. The body is not running ahead of itself. It is doing exactly what it was built to do.

The cortisol that arrives early

The cleanest way into this is a distinction most popular writing about stress skips entirely. We tend to model stress as reactive: the bad thing happens, the body responds. But a body can also respond before the bad thing. The two responses are measurably different.

A 2013 study in Psychoneuroendocrinology set out to separate them.[3] The researchers found that a real subset of people (somewhere between a fifth and two-fifths, depending on the protocol) showed cortisol rising before the stressor could have triggered any physiological response at all. Not in reaction. In anticipation. The body had begun mobilising on the strength of what it expected to come.

A field study a few years later took this out of the lab.[4] Students were measured before a written exam, a real one, with real stakes, in an ordinary setting rather than a controlled experiment. Around 37% showed the anticipatory pattern: cortisol elevated before any exam-related feedback had arrived. A smaller group reacted only once it began; a smaller group still did both. The point is not the exact percentages. The point is that for a substantial share of ordinary people, the stress response is already underway before the stressful thing exists in the room.

If you have ever felt most undone in the waiting (and then, oddly, steadier once the difficult thing finally started) this is the shape of what was happening.

A nervous system that predicts rather than reacts

There is a frame from computational neuroscience that makes sense of this, and it has not yet crossed into the usual wellness conversation. It treats the nervous system less as a reaction machine and more as a prediction machine.

A 2024 model approaches cortisol this way: as a signal the body uses to minimise the gap between what it expects and what it gets.[1] On this account, the system is not waiting passively to be acted upon. It is constantly forecasting the next demand and adjusting in advance, so that when the demand lands, the surprise (and the cost of the surprise) is smaller. I want to be careful here: this is a theoretical model, a simulation rather than a study of human bodies. But it gives language to something the measurements keep showing. The cortisol arriving early is not a malfunction. It is a forecast.

There is even a suggestion that the forecasting reaches into sleep. An intensive longitudinal study tracked people's evening expectations against the cortisol surge that occurs in the first half-hour after waking.[5] Evenings rated as more stressful-in-prospect were followed by a larger surge the next morning, as though the body had spent the night preparing for a day it had not yet had. I'd hold this one lightly: it followed only three participants, and it asks to be replicated before anyone leans on it. But it lines up with the Sunday-night-into-Monday-morning quality so many women describe: the sense of waking already braced.

A separate line of work has shown that the body's stress-readiness leaves traces you can read at rest, before any stressor appears: resting neural markers that track with cortisol and heart-rate variability across conditions.[2] I want to be exact about what that does and does not mean. It tells us the readiness state is real and measurable. It does not tell us your wearable can read tomorrow's dread in tonight's numbers. That prospective leap is not something the evidence supports, and I am not going to pretend otherwise.

Why this is not catastrophising

Here is the part that seems to matter most.

The Coper, the woman MoMo is built for, is unusually quick to read her own anticipatory tension as a verdict. The tightness the night before becomes evidence: that she is overreacting, that she cannot let things go, that a steadier person would not be lying awake over a meeting that has not happened. The science offers a different reading. The tension that arrives early is not a failure of perspective. It is a system reading the horizon and beginning to prepare, the same capacity that, in another light, we would call being ready.

And it is not fixed. A study of young women found that a more positive appraisal going in (being given reason to expect they could cope) produced a measurably lower cortisol response to the same stressor.[6] The forecast is not a sentence handed down. It moves with how the coming thing is held.

I'll resist turning that into a technique, because it isn't one, and the Coper has been handed enough techniques. It is closer to a fact worth keeping nearby: the way an upcoming thing is appraised feeds into how the body meets it. Not as a thing to perform correctly. Just as something the system is doing, with or without your supervision.

What to do with it, which is nothing

I wrote recently about the circuit that runs on sleep: what a short night does to the pause between feeling and reacting, the day after. This is the other end of it: the night before. There, the body was running degraded by what had already happened. Here, it is mobilising for what has not happened yet. Two different mechanisms, and worth not collapsing into each other.

One caution I'll keep honest. It is tempting to extend this into a longer story: that years of bracing accumulate into something that wears the system down. The allostatic model suggests as much, and it may well prove true. But the direct chain from Sunday dread to lasting damage is not something these studies have established, and I am not going to imply they have. What is established is narrower and, I think, enough: the body begins responding before the threat arrives, and that this happens does not mean anything is wrong with you.

You do not have to do anything with this. There is no evening protocol to follow, no number to bring down. But the next time the tension arrives ahead of the thing (the Sunday that turns before the week has touched it) it might be worth knowing that the body got there early on purpose. Reading the horizon is not the same as falling apart.

That recognition (the way a tension the night before connects to a difficulty that has not yet arrived, so it reads as a pattern rather than a personal flaw) is the kind of thread MoMo is built to hold across days. But that is a footnote. The essay is the early cortisol, and the nervous system that forecasts rather than merely reacts.

If this feels like more than a pattern (if you're in distress or crisis) please reach out: Mind Sweden 90101 | Samaritans UK 116 123 | your local crisis line.

References

Khan, I., & Lowe, R. (2024). Surprise! Using Physiological Stress for Allostatic Regulation Under the Active Inference Framework. arXiv preprint arXiv:2406.08471.
Maimon, N. B., Intrator, N., Gidron, Y., et al. (2025). Personalized Detection of Stress via hdrEEG: Linking Neuro-markers to Cortisol, HRV, and Self-Report. arXiv preprint arXiv:2509.13875.
Engert, V., Efanov, S. I., Duchesne, A., Vogel, S., Corbo, V., & Pruessner, J. C. (2013). Differentiating anticipatory from reactive cortisol responses to psychosocial stress. Psychoneuroendocrinology, 38(8), 1328–1337. PMID: 23246327.
Beltzer, E. (2022). An ecological approach for investigations of the anticipatory cortisol stress response. Biological Psychology, 175, 108428. PMID: 36181968.
Seizer, L. (2024). Anticipated stress predicts the cortisol awakening response: An intensive longitudinal pilot study. Biological Psychology, 192, 108852. PMID: 39102975.
Pulopulos, M. M., Baeken, C., & De Raedt, R. (2020). Cortisol response to stress: The role of expectancy and anticipatory stress regulation. Hormones & Behavior, 117, 104587. PMID: 31639385.