You Did Not Run Out of Energy. You Ran Out of Deciding.
Mind · The Reading Room, Vol. 9 · 9 min read · Why safety researchers separate fatigue from sleepiness, what brain imaging found underneath it, and one thing to do at two o’clock
You could not nap if someone paid you. You also cannot read the same paragraph a fourth time and retain any of it, which is a different problem wearing the same coat.
Two o’clock on a Wednesday and I read the same three lines of a contract four times. Not difficult lines. A date and a name and a number. The fourth time I got to the end and could not have told you the number. I did the thing everyone does, which is go and stand in front of the kettle for a while, and then come back and read it a fifth time.
What I was not, at any point in this, was sleepy. If you had put me on a sofa in a dark room I would have lain there wide awake, mildly irritated, thinking about the contract. What follows is a reading of published safety and neuroscience research on that state; I am not a clinician, the findings belong to the researchers named below, and only the plain-English version is mine.
The short answer: being mentally tired but not sleepy makes sense once you know these are two separate states that happen to share a vocabulary. Safety researchers who study this professionally treat fatigue and sleepiness as distinct, with different causes and different remedies.¹ Sleepiness is your body’s readiness to fall asleep. Fatigue is reduced capacity, arriving when what is being asked of you outruns what you have available. Sleep clears one of those. At two o’clock on a Wednesday you almost certainly have the other.
There is a name worth having for the thing that has actually run out, because it is much more specific than “energy”: the deciding budget. You are not out of alertness — you are demonstrably alert, which is the whole problem. You are out of the particular function that weighs options and commits, and the brain-imaging work now points at something measurable underneath it.²
The skim version
- NIOSH researchers describe sleepiness as sleep propensity: the inclination to fall asleep, measured by how fast you actually do.¹
- Fatigue they define as extreme tiredness with reduced functioning capacity, arising when demands outstrip available resources.¹
- The two have different origins. Sleepiness comes from short or disrupted sleep. Fatigue comes from overexertion and demand.¹
- They also have different remedies. Sleepiness keeps building until sleep happens. Fatigue can ease when the demand is removed or genuine recuperative rest is taken.¹
- A 2022 imaging study found that a full day of demanding cognitive work was accompanied by glutamate accumulation in the lateral prefrontal cortex, alongside a measurable shift towards low-effort, quick-payoff choices.²
- Which means a break can help one of these and do nothing at all for the other, and knowing which you have decides what to do next.
The source for this piece
Dr Tim Bauerle, PhD
An occupational health psychologist and safety and fatigue research scientist, and a co-author of the National Institute for Occupational Safety and Health paper this piece rests on. His field is not wellbeing writing — it is deciding who is safe to operate a machine at the end of a twelve-hour shift, which is why the definitions are unusually precise and why nobody in that literature is trying to sell you a routine.
What we read: Barham, Bauerle and Eiter, Mining, Metallurgy & Exploration (2023), alongside the Paris Brain Institute imaging study and the post-lunch dip literature.¹²³
Where to follow his work: LinkedIn · Society for Occupational Health Psychology profile · The paper itself
Two words that got treated as one
Three researchers at NIOSH published a short paper in 2023 with a title that reads like an argument they had already had several times: are fatigue and sleepiness the same.¹
Their answer is no, and their reason for writing it is not philosophical. They work in occupational safety, where the two get treated interchangeably, and where treating them interchangeably means people reach for the wrong control and the risk does not go down.
Sleep propensity is the technical term for sleepiness. It means the inclination to fall asleep, and it is measured by how quickly a person loses consciousness when given the opportunity. That is a real number, obtained in a sleep laboratory, not an opinion about how you feel.
Definition: fatigue, in this literature, is extreme tiredness accompanied by reduced functioning capacity, appearing when the demands on a person exceed the resources available to meet them. It is a statement about capacity, not about drowsiness, and the two can move in opposite directions on the same afternoon.¹
Read those two definitions next to each other and the two o’clock contract stops being mysterious. My capacity had gone. My sleep propensity had not moved at all.
Why this distinction changes what you do about it
Here is the part that made me sit back. The paper notes that sleepiness from inadequate sleep the night before may not be alleviated by work breaks, whereas work-related fatigue may be.¹ The authors put it more precisely still: sleepiness persists and increases in magnitude until adequate sleep is attained. Fatigue is alleviated by removing what is causing it, or by recuperative rest, which could include sleep but does not have to.¹
So the same intervention lands completely differently depending on which one you have. Ten minutes away from the desk is genuinely restorative for fatigue. For sleepiness it is a delay, and the debt keeps accruing whether you have a biscuit or not.
Which explains a small daily humiliation a lot of competent women report. You take a break. It does not work. You conclude the problem must be you, since a break is supposed to work and it did not, and you go back to the contract slightly worse off and now also embarrassed.
The break was fine. It was aimed at the wrong thing.
If your capacity is going by two most days, the interesting question is what has been drawing on it since seven. The Quiet Audit is a private ten-minute look at what is actually running in the background. Start here.
Which one are you actually having?
The test is quicker than you would expect, and slightly annoying, because it involves being honest about the sofa.
| Sleepiness | Mental fatigue | |
|---|---|---|
| Given a dark room | You would be gone in minutes | You would lie there awake and irritated |
| What it feels like | Heavy eyes, head nodding, losing the thread | Alert but blunt. Rereading. Small decisions feel large. |
| Where it came from | Short, broken or badly timed sleep | Hours of demand with no real gap in them |
| What actually helps | Sleep. Nothing else clears the balance. | Changing the demand, or genuine rest from it |
| If ignored | Keeps building until you sleep | Accuracy drops before you notice it has |
There is a genuine afternoon dip in alertness that most people have, and it is not caused by your lunch. Research on the post-lunch dip describes it as an endogenous rise in sleepiness, countered reasonably well by a short nap or by caffeine.³ If that is what is happening to you, those work. If you are blunt rather than drowsy, they will not, and the coffee mostly makes you a more alert version of unable to concentrate.
Between us — what the research found, and where it came from
The source I am leaning on is NIOSH, the American federal research institute for workplace safety, and the paper ran in a mining and metallurgy journal.¹ That is an odd door to come through, and I want you to know it is the door. The distinction it draws is standard in occupational safety and is not controversial.
The limitation, stated plainly: this work exists because somebody has to decide who is safe to operate a machine at the end of a twelve-hour shift. Nobody funded a version of it about a fifty-three-year-old with a contract, two teenagers and a mother in another county. The definitions are general and well established. The specific application to your Wednesday is mine, not theirs, and you should hold it that bit more loosely because of it. The same caution applies to the imaging study below: it is one experiment with a small sample, and a mechanism that is plausible is not the same as a mechanism that is settled.
Five Things That Look Like This and Are Not This
Everything above is about a normal afternoon in a demanding life. This part is the other possibility, and I would rather raise it than have you assume, for a second year, that it is just the workload.
Five ordinary conditions produce being blunt at two on days that were not even busy. Four of the five are settled by one appointment.
- Low ferritin, with or without anaemia. Common in the years before the final period. The tell is breathlessness on stairs and a bluntness that a good weekend does not shift.
- An underactive thyroid. Slowed thinking with cold intolerance, dry skin and weight change. One blood test.
- Obstructive sleep apnoea. It frequently presents as fatigue rather than as sleepiness, which is exactly why it gets missed by this test. Somebody else has usually noticed the breathing first.
- Medication. Beta blockers, some antihistamines, sedating painkillers and several antidepressants all blunt exactly this function. A prescription review takes five minutes.
- Depression. The distinguishing feature is not how heavy it feels but what happens on a genuinely clear weekend: depletion lifts a little, anhedonia does not — that is the responsiveness test.
None of these is ruled in by a two o’clock list, and none is ruled out by one. If the bluntness is there on quiet days as well as full ones, the list above is the appointment rather than the article — and a normal panel is itself a finding rather than a dead end.
Who Else Has Measured This
A definitional paper is a definitional paper. Two other groups have gone at the same thing with instruments.
Antonius Wiehler (ResearchGate · Scholar) and Mathias Pessiglione (Paris Brain Institute · Scholar · The Conversation) at the Paris Brain Institute ran the study I keep coming back to. They had people do a full working day of either hard or easy cognitive work, scanned them with magnetic resonance spectroscopy, and found glutamate accumulating in the lateral prefrontal cortex of the hard-work group. Alongside it, those participants shifted their choices towards options that were low-effort and quick to pay off.² That is the deciding budget, visible from the outside.
The post-lunch dip researchers — Mohamed Romdhani (ResearchGate · Scholar) and colleagues among them — establish the other half: there really is an endogenous afternoon rise in sleepiness, and a short nap or caffeine really does counter it.³ Two different afternoon problems, two different fixes, one shared complaint.
And the occupational safety literature generally has been working with this distinction for years, because in that world getting it wrong has consequences a productivity article does not have.¹
A definitional paper, a neuroimaging experiment and a sleep-laboratory literature, agreeing that these are two things. The disagreement is only ever about which one you are having.
The break was fine. It was aimed at the wrong thing.
What This Actually Changes
Not the workload. I am not going to pretend a list fixes a life that has two households running through it.
What it changes is the sentence at two twenty in the kitchen, which for most of us is some version of what is wrong with me lately. If the thing that has run down is specifically the capacity to weigh and commit, then the answer to “what is wrong with me” is: nothing that a different kind of task would not reveal. Ask that same brain to format a spreadsheet and it works perfectly.
It also changes where you spend your best hours. If deciding is the function that depletes, then the first two hours of the morning are not the hours that can absorb interruption — they are the hours you are actually spending. That is unwelcome news for anyone who has spent a decade treating the morning as buffer, and it is the most useful thing in this piece.
When this belongs with a doctor
Book an appointment rather than rearranging your afternoon if the bluntness is there on quiet days as well as busy ones; if a genuinely clear week does not move it; if you consistently cannot fall asleep when given the chance while still feeling wrecked; if it comes with breathlessness on stairs, palpitations, unexplained weight change, cold intolerance or a fever; if you wake unrefreshed after a full night or somebody has noticed your breathing in sleep; if you are losing words rather than losing focus; or if interest in things you used to want has gone flat rather than merely being crowded out.
What to Say, and What You Will Probably Hear Back
Two rooms, two versions. Both short enough to read off a phone screen, which is the only length that survives an actual conversation.
Short enough to read off a phone screen
At work, say: “My judgement is best before eleven. I would like the decisions in the morning and the admin after two. It is a scheduling change, not a capacity one.”
If you hear “can we just do it now”: “We can, and you will get my four o’clock answer. I would rather you had my ten o’clock one.”
At the appointment, say: “I am blunt rather than sleepy, and it happens on quiet days too. I would like ferritin, thyroid and B12 checked, and a prescription review, before we settle on stress.”
If the list of things drawing on your capacity before noon is longer than anyone knows, that is worth seeing on paper. The Quiet Audit is a private pass through everything you are currently holding. Start here.
What to do at two o’clock instead
Change the kind of work, not the location of it.
Mental fatigue is specific to what has been draining you. An afternoon spent deciding leaves you poor at deciding, and reasonably capable of things that are not deciding. So the move is to stop asking your worst function to carry the next hour.
In practice that means having a two o’clock list before two o’clock arrives, made in the morning when you were still sharp. Filing. Formatting. The expenses. The email that needs sending but not composing. Anything where the standard is done rather than right.
The reason it has to be written in advance is that choosing what to do next is itself a decision, and you have run out of those. I have watched myself stand in a kitchen at two twenty with a working brain and no ability to select a task, which is a strange way to lose twenty minutes.
The list has one rule that matters more than the contents. Nothing on it may be something you would feel proud of finishing. The moment a two o’clock task carries any weight, you will start doing it properly, which means judging it, which is the exact function that is currently unavailable. Boring is not a compromise here. Boring is the specification.
Questions people ask me about this
Why am I mentally tired but not sleepy?
Because they are two different states. Sleepiness is sleep propensity — readiness to fall asleep, measured by how quickly you do when given the chance. Fatigue is reduced functioning capacity, arriving when demands outstrip resources.¹ A day of heavy deciding produces the second without touching the first, which is why you can be wrecked and wide awake at the same time.
If I am not sleepy, does that mean my sleep is fine?
Not on its own. Poor sleep can present as fatigue rather than sleepiness, particularly if it is fragmented rather than short, and obstructive sleep apnoea often shows up this way. Consistently unable to fall asleep when you have the chance, while feeling wrecked, is worth taking to a clinician rather than solving with a better list.
Is this just what happens after fifty?
Some capacity change is normal, and midlife adds real load on top of it. What is not automatic is the conclusion that you have declined. Look at how many decisions your average Tuesday now contains compared with your average Tuesday at thirty-eight, including all the ones made on other people’s behalf. Most of the women I hear from have not slowed down. Their input has doubled.
Does coffee help?
For the genuine post-lunch dip, yes — caffeine and a short nap both counter it.³ For mental fatigue it mostly makes you a more alert version of unable to concentrate, because alertness was never the thing that ran out.
Why does this tiredness make me so hard on myself?
Because blunt gets filed as evidence about who you are, while drowsy would have gone under bad night’s sleep and been forgotten by Thursday. That filing move is worth catching. A judgement about your competence made at two o’clock comes out of a depleted system, which makes it a poor witness.
The one move
Tomorrow morning, while you are still sharp, write down three tasks for two o’clock that require no judgement at all. Then at two, do one of them without deciding anything.
Boring is not a compromise. Boring is the specification.
Where to Go Next
In order, and each for a reason.
1. If the rereading happens all day rather than only at two, part of your attention may still be on the last thing you switched away from — a different mechanism with a different fix.
2. Then the four different reasons focus goes at work, because they are genuinely four, and the advice for one makes another worse.
3. Then what a standard panel checks and what it never looks at, if the bluntness is there on the quiet days too.
You did not run out of energy. You ran out of deciding, and those refill differently.
References
1. Barham, M., Bauerle, T., & Eiter, B. (2023). Are Fatigue and Sleepiness the Same? A Brief Introduction to the Differences and Similarities and Their Implications for Work Safety. Mining, Metallurgy & Exploration, 41(1), 53–59. Read the paper
2. Wiehler, A., Branzoli, F., Adanyeguh, I., Mochel, F., & Pessiglione, M. (2022). A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. Current Biology, 32(16), 3564–3575. Read the study
3. Romdhani, M., et al. (2021). The Effect of Experimental Recuperative and Appetitive Post-lunch Nap Opportunities, With or Without Caffeine, on Mood and Reaction Time in Highly Trained Athletes. Frontiers in Psychology, 12, 720493. Read the study
On the researcher. Tim Bauerle is an occupational health psychologist working on worker fatigue and safety, not a clinician treating individuals, and nothing in his published work is medical advice. The 2023 paper is a definitional and safety-focused piece written for an occupational audience; the extension of it to an office Wednesday in midlife is ours, and is flagged as such in the box above.
Sourcing, disclosure and disclaimers
This is not medical advice. Blue Leaf Journal publishes general information for a general readership. Nothing here is a diagnosis, a treatment recommendation, or a substitute for care from a clinician who knows your history.
We are not clinicians. Blue Leaf Journal is an independent publication. We read published research and translate it. The findings belong to the researchers and institutions named and linked above; the plain-English rendering is ours, and so is any error in it.
No affiliation and no endorsement. Blue Leaf Journal is not affiliated with Tim Bauerle, Megan Barham, Brianna Eiter, Antonius Wiehler, Mathias Pessiglione, the National Institute for Occupational Safety and Health, the Centers for Disease Control and Prevention or the Paris Brain Institute. None of them has reviewed, approved or endorsed this article, and none of them is responsible for it. They are cited because their published work is the evidence for what it says.
No commercial relationship. Nobody named above paid for or was paid for this coverage. There are no affiliate links, no sponsored placements and no gifted products in this article.
How this was checked. Every claim above is drawn from the primary sources, each linked in the references, and can be verified there. Where the extension from the research to a midlife working day is ours rather than the authors’, we have said so in the body. Sources were checked on 27 August 2026. If you find something we have got wrong, write to jobrennan@blueleafjournal.com and we will correct it and say that we did.
Jo Brennan writes the Mind and Load desks at Blue Leaf Journal, mostly about the parts of a competent life that nobody puts on a job description. She now writes her two o’clock list at nine in the morning. It is currently three items long and one of them is “water the plant,” which she has done four days running, and the plant is thriving under a system designed entirely around her own decline.
Written by Jo Brennan for Blue Leaf Journal. Updated: 27 August 2026.
