What Mental Fatigue Actually Does. Part One: What Shows Up in the Body

Abstract

Everyone who has worked a long day at a screen knows the feeling: the work gets slower, errors creep in, and the effort of continuing rises even though nothing physical has happened. The question this series takes seriously is whether that experience corresponds to anything measurable — and if so, what.

This first part examines a 2025 systematic review and meta-analysis by Goodman and colleagues in Behavior Research Methods, which pooled seventy-two laboratory studies of mental fatigue and asked a deliberately unglamorous question: across all this work, which bodily measurements actually move?

The answer is more mixed than the popular account suggests. Cardiovascular measures move reliably. Brain-wave measures move inconsistently. Several markers that feature prominently in commercial claims barely move at all. We also think the paper's most valuable contribution is not its results but its verdict on the field's methods.

1.Why Someone Bothered to Pool Seventy-Two Studies

Mental fatigue is unusual among research topics in that almost everyone accepts it exists while almost nobody agrees on how to measure it. Goodman and colleagues describe it as a transient psychophysiological state marked by impaired cognition and behaviour, and note that despite growing interest, its physiological representation remains unclear.

That vagueness has a practical cost. If a laboratory wants to test whether some intervention reduces mental fatigue, it must first decide what to measure. Heart rate? Brain activity? A questionnaire? Each choice produces a different study, and studies that measure different things cannot be compared.

The review set out to map that terrain. Seventy-two studies met the inclusion criteria — sixty-one identified initially, with eleven added subsequently — yielding thirty distinct physiological outcomes, four visual outcomes, and several neuroimaging approaches. That inventory is itself the first finding: a field investigating one phenomenon through thirty-four different windows.

2.What Reliably Moves: The Cardiovascular Signal

The clearest results concern the heart and circulation. Compared with control conditions, mental fatigue increased heart rate, systolic and diastolic blood pressure, and mean arterial pressure. Several indices of heart rate variability also shifted: low-frequency power rose, RMSSD rose, and SDNN fell. All of these reached statistical significance at P ≤ 0.04.

This deserves unpacking, because heart rate variability is frequently invoked in consumer wellness contexts with more confidence than the underlying data support. The measure describes the variation in time between successive heartbeats, and different statistical treatments of that variation are thought to index different branches of the autonomic nervous system.

The pattern here is not simple. Some variability indices rose while another fell, which is not what a single clean autonomic shift would produce. The honest reading is that sitting still and concentrating hard for an extended period changes cardiovascular regulation in a detectable and reproducible way, and that the direction of that change is not captured by any one number.

3.What Moves Inconsistently: The Brain-Wave Measures

Electroencephalography was a popular choice among the studies reviewed, and it is where the picture becomes murkier. Theta activity — the slower rhythm often linked to drowsiness and to control processes in working memory — rose in some studies at some electrode sites, and did not change in others.

The reported increases were regionally specific rather than global. One study found elevated theta at a frontal site during a rapid visual information processing task; another reported increases over the somatosensory association cortex and angular gyrus. In several other studies theta was unchanged.

Alpha activity, monitored in most of the studies and sometimes split into lower and upper bands, was mostly unchanged by mental fatigue. Two studies out of several found an effect. That is close to what one would expect from chance alone, and the review does not oversell it.

The authors nonetheless argue that EEG is worth including in future work, while flagging a confound that is easy to overlook: circadian and homeostatic processes produce EEG fluctuations on their own. A study that runs its fatigue condition in the afternoon and its control in the morning has built the confound into the design.

4.What Barely Moves At All

Among the visual measures — gaze fixation and fixation order, pupil diameter, blink rate — only a single study reported that pupil diameter was reduced by mental fatigue. Another study monitored pupil diameter across two task types and found no difference at all.

We highlight this because pupil response and blink rate are attractive measures: they are non-invasive, they can be captured by a camera, and they are increasingly proposed as the basis for consumer fatigue tracking. The pooled evidence for them as markers of mental fatigue is, on this review's showing, thin.

Cortisol appears in the literature but in a more interesting role than a simple marker. In one study, participants were grouped by whether they mounted a cortisol response at all, and the brain-activity differences under mental fatigue appeared in the non-responders — greater activity across prefrontal, temporo-parietal and parietal regions during encoding and recognition. No comparable changes appeared in the control condition for either group. Whatever that means, it is not a story of one hormone tracking one state.

5.The Detail That Tells You This Was Done Carefully

Meta-analysis requires converting many different studies into a common currency, and the choices made in that conversion can determine the answer. Goodman and colleagues did something we rarely see: they ran the analysis twice.

One version used the most conservative effect sizes available from each dataset — those closest to a null finding. The other used the least conservative, the most pronounced effects. Reporting both is a way of showing readers how much the conclusion depends on a judgement call that would otherwise be invisible.

The effect magnitudes themselves were graded by standard convention: trivial below 0.20, small from 0.20 to 0.49, moderate from 0.50 to 0.79, and large at 0.80 or above, expressed as Hedges' g with confidence intervals. Between-study heterogeneity was assessed with Cochrane's Q at a threshold of P ≤ 0.10 and graded with the I² statistic across bands from low to considerable.

None of this is exciting to read. It is what separates a synthesis you can rely on from a list of studies that agree with each other.

6.The Real Finding: The Field Cannot Agree on How to Cause the Thing It Studies

Running underneath the results is a problem the authors state directly: heterogeneity exists in the type of task used to induce mental fatigue in the first place. Before any measurement happens, the researcher must make the participant mentally fatigued, and there is no standard way to do it.

Different laboratories use different tasks of different difficulty for different durations. If one study fatigues people with twenty minutes of a colour-naming task and another with two hours of puzzles, the two are not studying the same intervention, and pooling their physiological outcomes assumes a comparability that has not been established.

The authors conclude that despite seventy-two studies, further investigation is required across a range of methodological factors. Read plainly: the evidence base is large but not yet standardised enough to settle the question. That is a finding about the state of a science, and it is the reason we are treating this topic as a series rather than a single confident article.

7.What a Reader Should Take From This

Three things, in our reading. First, the experience of mental fatigue does correspond to measurable bodily change, most reliably in cardiovascular regulation. It is not merely a mood or a failure of discipline.

Second, no single measurement currently deserves to be called the marker of mental fatigue. Anyone offering a device or a test that claims to quantify your mental fatigue from one signal is ahead of this evidence.

Third, and least comfortable: because the field has not standardised how it induces the state, results across studies are harder to combine than the volume of publications suggests. The next part of this series looks at one experiment that handled this problem unusually well — and at another that reached the opposite conclusion about whether mental fatigue impairs performance at all.

Editorial Comment

MindHeaven® makes no claim that any product reduces mental fatigue or alters any of the physiological measures described here. The evidence reviewed above concerns what the state looks like when it is induced in a laboratory, not what shifts it. We publish this because mental fatigue sits directly behind the problems our readers describe, and because it is a category where confident marketing has badly outrun the science.

One practical note, and it matters more than the rest. Everything above concerns transient fatigue induced by a demanding task in healthy volunteers, and none of those studies tested a nutrient. Everyday tiredness is a broader thing, and it does have a nutritional component: diet quality drives micronutrient status, and shortfalls in several nutrients present as tiredness. That is the one part of this territory where the evidence is settled enough to have been authorised. Niacin and vitamin B6 each contribute to the reduction of tiredness and fatigue — an authorised health claim under Reg. (EU) 432/2012 — and Boost supplies them at 125% and 100% of the nutrient reference value.

What a supplement cannot do is tell you which kind of tiredness you have. Exhaustion that does not lift with rest has causes worth identifying — low ferritin, thyroid function, a sleep disorder, depression — and several of them are corrected by something other than a supplement. Finding the source is worth more than managing the symptom, which is why we would rather you had that conversation with a doctor first and then decided what, if anything, to add.