What your brain scan looks like after 6 hours of decisions

You'd get bloods done if your energy tanked. You'd check your iron, your thyroid, your cortisol. You’d go for an assessment with a doctor or physical therapist and come up with a plan if you had persistent aching in your back. But when your brain hits a wall midway through your day, when simple decisions feel effortful, when you read the same paragraph three times, when you default to "I don't care, you pick" — you just push through and blame yourself later.

What if there was a scan that showed you exactly what was happening inside your head at that moment? Not a feeling. A measurable, physical marker, building up in the one brain region involved with every decision you make. And what you don’t realise is that the decline you feel? It starts building long before you’re consciously aware.

A group of neuroscientists looked into the physiological markers inside the brains of people under prolonged cognitive load – and I’m going to tell you what they found.

Once you understand, you'll stop blaming yourself. And you'd probably start taking it a lot more seriously.

What neuroscientists observed 

In 2022, a team at the Paris Brain Institute put people in brain scanners over six hours of cognitive load. They weren't measuring brain activity. They were tracking brain chemistry in real time, using magnetic resonance spectroscopy.

What they found: in the group doing hard cognitive work, a neurotransmitter called glutamate was building up in the lateral prefrontal cortex, aka the region responsible for your decision-making, planning, and complex reasoning. The harder the work, the more glutamate accumulated. And only this region showed the buildup, not the visual cortex used as a comparative region, which was equally active the whole time.

As the glutamate accumulated, behaviour changed. Participants invested measurably less cognitive effort into their decisions. They started systematically choosing easier, faster, more immediately rewarding options, even when a better option was right there. The brain was specifically gravitating toward whatever required the least effort, because the system responsible for pushing toward the harder, better choice was chemically compromised.

That's what the 3pm "I don't care, just pick something" moment actually is. Not apathy. Neurochemistry.

The part that should unsettle you

Both groups: hard cognitive work and easy, reported feeling equally tired. No significant difference in subjective fatigue ratings. The people who had significantly greater glutamate accumulation, and whose decision-making had measurably degraded couldn't feel it happening. The researchers noted that subjective fatigue reports are "notoriously unreliable." Your sense of how tired you are doesn't track with what's actually going on inside your brain.

I really resonated here. Not just as a neuroscientist, but as someone who has lived it.

I remember going in for one of my neuroscience exams. I was so ready. Weeks of preparation, top of my class, full of confidence, certain I was going to ace it. It was a long exam, 4 hours, I felt pretty sharp the whole way through. I got my marks back several months later. I don’t even need to tell you what happened. Can you imagine the gap between the marks in the first half of the paper compared to the second?

We found the same thing

In Numin's clinical trial — we saw this exact pattern. Subjective fatigue scores between the Numin and placebo groups? Not statistically significant. If you'd asked them, both groups would have said they felt the same level of fatigue.

But performance told a different story. The placebo group's win rate declined across the day. Their team communication dropped. Their competitive margin eroded. The Numin group's didn't. Same perceived fatigue. Statistically significant, measurably different output. 

That's the problem with decision fatigue. And it's also the opportunity. Because if the decline is biological, the support can be biological too.

One thing you can do today

Front-load your most consequential decisions to the first few hours of your day, before glutamate accumulation reaches the point where your prefrontal cortex starts pulling back. That email requiring a nuanced judgement call? Morning task, not afternoon. The research is clear: the decisions you make earliest will be your sharpest. Don't spend that window on inbox triage.

And if you want to extend that window, that's what Numin was designed for. Not to override the biology. To support it.

The brain scan doesn't lie. Now you know what yours looks like after you load it.