Hi! Welcome. You being here means more than you know. Knowing it lands with someone like you keeps me going. I'm Lavena Xu-Johnson. I write about psychology for founders. Why? Because scaling a business means scaling ourselves first.
Mirror, mirror on the wall, who has the most long-lasting, productive, efficient brain of them all…
One night last week, this random phrase popped into my head.
I was exhausted, not physically, or emotionally, but brain-hausted…
After having a burnout in 2021, I know the last thing I want is to experience it again. ‘Accidental’ overworking is a habit most entrepreneurs have slipped into, even though we know it is completely counterproductive in the long run.
In the past year, I have experimented with all kinds of small ways to protect my energy and resources, leading to healthier, more sustainable workdays. But I have not dug into the root of what I am protecting.
In this issue, we uncover some fascinating findings about the origins of cognitive fatigue from a 2022 neuro-metabolic experiment at the Paris Brain Institute.
Let’s jump in.
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How does your brain’s battery work?
There is a common assumption that willpower should be consistent throughout the day and is largely a matter of discipline and intention.
But what is the role of mental capacity in that assumption?
In 2022, a research team at the Paris Brain Institute conducted an experiment that provided direct neurochemical evidence of cognitive fatigue in the brain.
They recruited two groups of participants: one assigned to high-cognitive-demand tasks for a full working day, the other to low-cognitive-demand tasks, and used magnetic resonance spectroscopy to measure neurotransmitter concentrations in the lateral prefrontal cortex throughout the day.
By the end of the day, the high-demand group showed measurably elevated glutamate levels in the lateral prefrontal cortex. The low-demand group did not.
Wiehler et al. (2022) proposed that this glutamate accumulation is the brain's chemical signal of genuine cognitive fatigue: a marker that the prefrontal cortex's capacity to sustain effortful work is neurochemically constrained.
The finding suggests that cognitive fatigue is not a mood state or a lack of motivation, but a change in the biochemical environment of the brain region responsible for planning, judgment, and complex decision-making.
Both groups were also given a series of economic decisions at different times of the day, and by the end of the day, the two groups were choosing very differently:
High-demand group (hard tasks all day):
Preferred smaller rewards available immediately over larger rewards available later
Chose easier tasks even when harder tasks offered more money
Showed a consistent drift toward low-effort, low-stakes options as the day progressed
Low-demand group (easy tasks all day):
Maintained stable preferences throughout the day
Continued to choose larger delayed rewards over smaller immediate ones
Remained willing to take on harder tasks when the reward was proportionally greater
A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. By Wiehler et al. 2022.
What is happening in the brain when you feel ‘brain-dead’
To understand why this happens, we need to start with what the brain is metabolically doing during demanding work.
The brain accounts for roughly 20% of the body's total energy consumption despite representing only about 2% of its mass. The majority of that energy maintains the baseline electrochemical state of billions of neurons.
The prefrontal cortex (PFC) is the most metabolically intensive region to operate at full capacity. It is responsible for high-demand cognitive work, making complex decisions, managing competing priorities, switching between tasks, or even suppressing emotional reactions. Each one of these processes requires sustained, energetically costly neural signaling.
Glutamate is the brain's primary excitatory neurotransmitter, the chemical that activates neurons and enables cognition. As this signaling continues for hours, glutamate accumulates in the synaptic space of the lateral prefrontal cortex at levels that exceed its recycling capacity, thereby impairing the very circuits it targets and potentially becoming toxic to neurons. In response, the brain reduces prefrontal cortex activity to protect against further metabolic stress. The result is a gradual, measurable degradation in the quality of the cognitive work the prefrontal cortex can sustain.
This is what we experience as the 3 p.m. wall, or the specific numbness that we feel after a full day of back-to-back high-stakes conversations.
Too much neuroscience? Here’s what it means in plain terms:
Imagine your prefrontal cortex is a kitchen that produces meals all day. Cooking creates waste: dishes, scraps, mess. Normally, the kitchen cleans as it goes. But if you cook non-stop at high intensity, the cleaning can't keep up with the mess, and eventually the kitchen starts slowing down production to avoid total chaos.
The glutamate is the mess. The prefrontal cortex is slowing down because the kitchen is managing the backlog, so it prioritizes easier choices - ordering takeaway instead of cooking.
What does your 4pm brain actually do?
3 things that deplete cognitive energy faster
Not all cognitive work depletes at the same rate. There are three variables that are particularly relevant.
Task switching:
Every time we interrupt high-demand work to handle something unrelated, the PFC rebuilds its working memory state from scratch when returning.
Research on task switching suggests this overhead is substantial: the cost is not just the interruption itself but the re-engagement that follows it. A day of fragmented attention depletes faster than the same number of hours spent in sustained focus, even if the total volume of work looks identical.
Emotional regulation:
Adjusting tone in a difficult conversation, maintaining composure under pressure, or sustaining patience through a frustrating meeting all draw on the same executive circuits as analytical cognition.
The PFC consumes energy from the same resource pool and does not maintain separate accounts for emotional labor and strategic thinking.
Unresolved problems:
Holding multiple unresolved decisions simultaneously: the questions that sit at the back of our mind between meetings create a persistent low-level load on working memory.
The PFC stays partially engaged on those problems even when we are not consciously attending to them, contributing to the depletion curve throughout the day.
How to protect your cognitive resources
1. Sequence by cognitive demand
The first 90 to 120 minutes after full waking are typically the period of lowest glutamate accumulation in the PFC: the cleanest cognitive state of the day.
That window is expensive to spend on email, approvals, or low-stakes coordination. Reserve it for the decisions that require the most effortful, original thinking.
2. Treat rest as a neurochemical recovery need, not laziness
The glutamate accumulation described in the Wiehler data requires genuine disengagement from high-demand processing to clear out.
A break spent on another demanding task does not count (this is where I personally always get it wrong). Try open-ended, low-stimulus attention activities: a walk, a non-structured conversation, or time without cognitive input (not doomscrolling, either!).
3. Account for emotional labor as cognitive spend
If the morning has included a difficult conversation or an intense pitch that left you frustrated, you probably deserve a brain break at midday more than you realize, to sustain longer working hours later if needed.
4. Pre-commit decisions at the end of a workday
Because a fatigued brain reliably defaults to familiar, low-effort options, decisions that matter most should not be made at the end of a loaded day. Set boundaries, pause, and simply reply: "I will let you know first thing in the morning.”
As always, hit reply if something in here hits home.
Until next week,
Lavena
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