Anxiety is treated in one place, chronic pain in another, sleep problems in a third, and cognitive complaints somewhere else again. The division is administrative rather than biological, and one of the more striking features of the recent literature is how often the same measurements turn up regardless of which door a person came through. That recurrence is worth taking seriously. It is also worth being careful about, because the obvious conclusion from it is wrong.

1. Fatigue Looks the Same Wherever It Appears

A 2025 meta-analysis took a deliberately indiscriminate approach: it included studies of any medical condition in which patients showed significantly higher fatigue than healthy controls. Forty-six studies, 2,603 individuals.

What emerged was a widespread cortical–subcortical network — frontal, limbic, basal ganglia and parietal — organised into three clusters (Brain Communications, 2025).

A signature that recurs across unrelated illnesses is telling you something about the symptom rather than about any one diagnosis. The authors also report a moderate risk of bias in most included studies, which is the appropriate brake on how firmly to hold it.

2. Biological Aging Markers Recur Too

A 2026 case-control study assembled structural MRI from 45,900 healthy controls and 2,698 patients across nine disorders, and compared the gap between MRI-predicted and actual age.

It was consistently greater across disorders, ordered by family: largest in dementia, then addiction, then the psychiatric conditions (PLoS Medicine, 2026).

Three quite different families of condition, one measure, same direction. That is the second recurrence.

3. The Body's Signals Are Part of the Picture

Two threads run in the same direction. In persistent pain, central sensitisation describes altered processing that explains the mismatch between tissue damage and pain intensity — the experience outrunning what the periphery accounts for.

In anxiety and depression, the insula’s job of interpreting internal bodily signals is central enough that imaging and behavioural studies of interoception have become a literature of their own.

In both cases the interesting action is at the interpretation stage rather than at the point of detection, which is a structural similarity between conditions that look nothing alike from the outside.

The reason this counts as a cross-cutting theme rather than two separate facts is the shared shape. In both cases the periphery is reporting something, and what varies between people is what happens to that report on the way to becoming an experience. A system whose interpretive stage has shifted will produce different experiences from identical input — which is a mechanism available to conditions that have nothing else in common.

4. Sleep Sits Underneath Nearly All of It

Sleep is the most consistent cross-cutting thread in this whole set of articles. Modest sleep restriction changes children’s behaviour measurably. Circadian instability after trauma tracks with later symptom severity. Trauma-associated sleep disturbance is strongly associated with symptoms of PTSD, depression and anxiety simultaneously.

It is not that sleep causes these conditions. It is that a disturbance in one system shows up in the others, reliably enough that measuring it tells you something about all of them.

It is also the most practically useful of the recurring measures, for an unglamorous reason: it is the one that can be observed continuously, at home, without a scanner. A signal that appears across conditions and can actually be measured in ordinary life is worth more than one that requires a research facility.

5. And the Exception That Makes the Rule Honest

Here is the finding that stops this from becoming a slogan. In that same study of nine disorders, the developmental disorders were not different from expected. ADHD and autism did not show the elevated brain age gap that the dementia, addiction and psychiatric groups did.

If the story were simply that all conditions accelerate the same underlying process, that result should not exist.

So the recurrence is real and it is bounded. Some measures cut across many conditions; at least one cuts across some and not others — and that pattern of exceptions is more informative than the recurrence alone.

It is also a useful test of any claim in this area. An account that explains everything equally well is usually explaining nothing, and the version worth trusting is the one that predicts where the pattern stops.

What Shared Measurements Do Not Mean

They do not mean a shared cause. Conditions can converge on similar downstream consequences from entirely different origins. Fatigue accompanying multiple sclerosis and fatigue accompanying depression can involve overlapping networks without arising the same way.

They do not mean the categories are useless. Diagnoses carry real information about course, risk and what tends to help. Noticing that boundaries are permeable is different from concluding they are arbitrary.

They do not license a single explanation. Inflammation, sleep, stress and metabolism all appear across this literature, and each has been offered as the unifying mechanism by somebody. None is established as such.

And they are not diagnostic. Every finding described here is a group-level average with wide individual variation, from cross-sectional comparisons that cannot establish direction.

What It Does Support

That the boundary between mental health and physical health is administratively convenient and biologically porous — and that this is an empirical observation from large datasets rather than a philosophical position.

That measuring things which cut across categories, particularly sleep, autonomic state and fatigue, tells you something regardless of which diagnosis is in play.

And that a system under sustained load tends to show it in more than one place at once — which is why what happens overnight and what the body reports before the mind does keep appearing in conversations that started somewhere else entirely.