scientists figure out why thinking too hard for too long makes you tired

How could you possibly be tired sitting behind a computer all day? Because as it so turns out, your brain can chemically fry itself if you overthink.

In discussions where work fatigue turns into a competition, you’re bound to hear the following from a person who does a lot of physical labor: “How tired can you be sitting behind a computer all day, looking at a screen? I have to bench press trucks all day for a living! You don’t even know what tired is!” There are exactly two problems with this train of thought. First, this is not a competition and all jobs can be taxing in their own way. Second, scientists just so happen to know for a fact that mentally draining tasks will make you physically tired, and office workers who spend long days at their offices or behind their screens do end their days drained and exhausted.

But for as long as scientists have known this, they’ve been wondering why you pay a physical cost for mental labor. A new study finally offers a possible answer. Glutamate accumulating to near-toxic levels in your prefrontal cortex and your brain triggering a fatigue response in your body to get you to back off before those levels increase even further. What is glutamate? It’s a neurotransmitter used in memory and learning, and it makes neurons more likely to fire, which is a requirement to actually make use of your skills to complete mentally taxing tasks. Unfortunately, there’s a chemical tradeoff for using it, and that’s where physical fatigue comes in.

Glutamate starts as glutamine in your glial cells, the white matter in your brain that’s responsible for all the maintenance and cleanup work of your brain but gets none of the glitz and glamour of its neuron cousins. Its job, as already noted, is to excite the right neurons making them more likely to fire by latching on to their synapses. Now, in normal conditions, this is exactly what you want to happen because that helps make a lot of seemingly random pathways in your mind truly useful. But too much glutamate means the synapses keep on firing until they start to break down, and if this becomes a brain-wide problem, it quite literally chemically fries your mind.

how way too much of a good thing hurts you

Excess glutamate is found in Alzheimer’s, Huntington’s, and Parkinson’s patients, and while it’s not necessarily a main cause of any of the aforementioned illnesses — those would be excess tau proteins, genetic defect, and pollution, respectively — it tends to be an aggravating factor. In other words, it’s often an indicator of neurodegenerative disorders and it’s very possible that your body evolved ways to signal that its levels in certain parts of the brain need to be reigned in. And it’s exactly this that the study we noted above tried to measure by seeing how much they could tire out subjects with tasks requiring complete mental concentration.

While a control group flagged what letters swapped colors and grouped consonants and vowels together, an experimental group had to do the same tasks on hard mode, almost nonstop for over six hours. The researchers then measured commonly used metabolic and physical indicators of physical exhaustion, which included resonance scans of their brains’ metabolic signatures. The group given harder tasks was more physically tired, objectively speaking, and had a notably higher buildup of glutamate in the part of the brain responsible for decision making and executive function which coordinates cognitive workload: the prefrontal cortex.

Of course, this is not a definitive result. There were only 40 participants and there’s quite a bit to look into further to confirm the initial findings. But at the same time, the data does seem to fit the often cited observations that after about five or six hours of intense focus and thought, or deep learning, people tend to feel drained and start to make a lot more mistakes. Given the metabolic profile of the test subjects who hit the same wall, it does make sense that fatigue is the body’s warning to stop before we fry something important in the brain as our overtaxed prefrontal cortices struggle to keep up with the demands imposed on us, increasing our error rates.

how your body defends itself from, well, you

There’s another reason why this hypothesis seems logical. It’s similar to how much of your body behaves under strain. For example, during physical exertion, your muscles break down carbohydrates and produce lactic acid. Build up enough and you start to feel tired and achy, a signal to back off to stop the acid’s buildup beyond safe levels and into lactic acidosis which can cause cramps, nausea, dizziness, confusion, and in extreme cases, damage to the liver and kidneys. Pushing even past that point could start to break down and kill muscle, sending the tissue into the bloodstream, leading to a kidney-killing condition called rhabdomyolysis.

Simply put, your body has limits to how far you can push it and expect it to function at peak performance or heal any damage your efforts inflicted. Cross those limits with too much physical activity or taking on too much shock, trauma, or exertion, and your body will scream at you to ease off before you cause permanent or long term harm. If your organs and muscles can sound an alarm, why can’t your brain as it’s being slowly cooked by the demands you’re putting on it? It’s in your body’s interest to have some sort of chemical trigger to step in and try to prevent the worst possible outcomes so you survive as long as possible and successfully pass on your genes.

And all this prompts the question of why so many of us keep burning ourselves out if not physically then cognitively, training ourselves to ignore every fiber in our beings screaming at us to stop, for the love of all that’s cute and fluffy, before we do serious long term damage to our minds and bodies. Gaining some appreciation for our limits and finding the best ways to time and structure our work to avoid regular overload is what studies like this are ultimately all about, and we should take them seriously if we want to have longer and healthier lives rather than end up with ulcers, bad joints, and fuzzy minds in what are so generously called “our golden years.”

See: Wiehler, A., Branzoli, F., Adanyeguh, I., et al (2022) A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions, Current Biology, DOI: 10.1016/j.cub.2022.07.010