You forget a name thirty seconds after hearing it. You lose the thread of your own sentence mid-way through. You sleep a full night and wake as tired as when you went to bed. Somewhere along the line you started calling this “just being busy.”
It is not a character problem. Allostatic overload and brain fog describe what happens when the system managing your stress response stops keeping pace with the demand placed on it — and unlike tiredness, that process leaves marks you can measure.
What allostatic load actually means
Allostasis is the body’s continuous work of staying stable while conditions change: adjusting heart rate, cortisol, glucose and inflammatory signalling minute by minute. Allostatic load is the cumulative cost of doing that work without adequate recovery.
The World Health Organization classifies burnout in the ICD-11 as a syndrome resulting from chronic workplace stress that has not been successfully managed. That places it as a factor affecting health status rather than a standalone illness — but classification says nothing about whether the underlying biology is real. For the most direct evidence of that, including MRI-confirmed structural brain changes and how much of it reverses, see our piece on cortical thinning and burnout.
What those authors contributed was parsimony. They found that an index of five biomarkers available across every cohort — C-reactive protein, resting heart rate, HDL cholesterol, waist-to-height ratio and HbA1c — predicted mortality “as well or better than more elaborate sets of biomarkers.” Five markers, not thirty.
The vagus nerve and the cardiovagal brake
Your autonomic nervous system runs two opposing branches: the sympathetic, which accelerates, and the parasympathetic — carried largely by the vagus nerve — which brakes. We go deeper on this mechanism, including its direct role in inflammation, in our piece on the vagus nerve and the cholinergic anti-inflammatory reflex.
Heart rate variability is the most accessible window into that balance. It measures small variations in timing between consecutive heartbeats. Counter-intuitively, more variation is generally healthier: it indicates a responsive braking system. A metronome-steady heart rate suggests the brake has stopped engaging.
Kane and colleagues at the University of Aberdeen conducted the most rigorous review of this to date: a preregistered systematic review and meta-analysis of continuous 24-hour HRV monitoring in doctors, published in Occupational Medicine in December 2025. From 805 records they identified seven eligible studies totalling 176 participants across seven countries.
| Parameter | Studies | n | Hedges g | P | I² |
|---|---|---|---|---|---|
| SDNN | 4 | 63 | −1.05 | 0.001 | 64% |
| RMSSD | 3 | 43 | −0.63 | 0.005 | 0% |
| LF/HF ratio | 2 | 33 | 0.69 | 0.006 | 0% |
| LF | 3 | 43 | 0.54 | 0.01 | 0% |
| HF | 3 | 43 | −0.24 | 0.28 — not significant | 5% |
Highlighted row: high-frequency HRV — the parameter most specifically reflecting parasympathetic activity — showed no statistically significant change between stress and recovery.
Two findings there deserve attention that you will rarely see quoted. High-frequency HRV showed no significant change. And the largest effect came with the widest uncertainty— SDNN produced the strongest result but also the only substantial heterogeneity in the set.
What the researchers themselves say about the limits
This is the section most articles omit, and it is the most useful one here. The authors state their conclusion plainly:
It is not possible to draw any meaningful clinical conclusions.
They add that “little is known about what the minimum meaningful clinical difference in HRV would need to be between stress and recovery to positively reduce burnout risk.” In other words: HRV detects the difference between a stressful shift and a rest day. Nobody yet knows how much your HRV needs to improve before your burnout risk actually falls.
Other limits worth knowing: the constituent studies ran from 2009 to 2018, every one used a different measurement device, monitoring lasted only 24 to 48 hours, and none was conducted in the UK.
Want the full evidence review?
Six biomarker systems, a reflective self-check, and the lab targets to raise with your clinician.
The population problem
Almost all of this research was conducted in clinicians. Anaesthetists, surgeons, obstetricians, emergency physicians. Search the burnout–HRV literature and the same populations appear repeatedly.
There are practical reasons. Hospitals are where researchers already work. Shift patterns create clean natural experiments with defined stress and recovery windows. And clinician burnout is a documented crisis — the 2023 UK General Medical Council survey of over 45,000 doctors found 66% of trainees and 52% of trainers reporting moderate to high burnout.
So what does that mean for a lawyer, a founder, an engineer, a parent running on empty?
The mechanisms — vagal withdrawal, cholinergic anti-inflammatory signalling, glucose dysregulation under sustained sympathetic drive — are general human physiology. They are not profession-specific. The magnitudes are another matter. A twelve-hour surgical rotation is not the same stressor as eighteen months of unrelenting deadline pressure, and it would be dishonest to pretend the effect sizes transfer cleanly.
The Kane authors note this themselves: their studies compared discrete high- and low-stress periods over 24 to 48 hours, whereas “chronic stress and recovery imbalances contributing to burnout are likely to accumulate over much longer periods of time.”
The metabolic link: HbA1c
HbA1c reflects average blood glucose over roughly three months. It earns its place in McCrory’s five-item index, which puts it among the markers that carry real predictive weight. The mechanism is straightforward: sustained sympathetic activation mobilises glucose for a physical response that never arrives. Sustain that for months and regulation drifts. We go deeper on this marker, including where the burnout-specific evidence gets genuinely mixed, in our HbA1c piece.
4.0% – 5.6%
Standard homeostatic range
≥ 5.7%
Threshold flagging metabolic dysregulation
If you encounter a figure below 4.0% quoted as a “target,” it is an error. Values that low sit outside normal human physiology.
What you can actually measure
You cannot manage what you have never looked at. Three markers are worth discussing at your next appointment: HRV, watched as a trend across weeks rather than any single night; HbA1c, a routine blood test; and CRP, a standard inflammatory marker and one of McCrory’s five.
None of these diagnoses burnout. There is no blood test for it. What they do is convert a vague sense of “I’m not right” into something with a trajectory that you and your doctor can watch over time.
Start with a baseline
The Burnout Biomarker Checklist
A 13-page evidence review covering six biomarker systems, a reflective self-check, and the specific lab targets to raise with your clinician. Free.
Start with the Self-Check on page 11. It takes about four minutes.
The Capio Initiative is an educational and resilience-training platform. We are not a medical practice. The biomarkers discussed are for physiological understanding and burnout recovery, not diagnostic criteria for medical disease. Speak with a qualified healthcare professional before changing anything about your recovery, training, or use of tracking hardware.
