Not all in your head: New answers on the chemistry of mental fatigue

I’ve always found it irritating that mental effort can be so exhausting. As someone whose work has revolved around reading, thinking, and writing for over a decade now, it never ceases to annoy me the way my brain will sometimes just shut down when I try to do work. I’ve found it to be a major obstacle k for as long as I can remember. I’ll try to re-focus on something that I want to get done, and my I’ll suddenly have a hard time even staying awake. It’s not every time, of course, but it’s often enough to be annoying, and it does tend to happen more when I’ve been working hard for a few days.

I’ve never really thought much to the brain chemistry behind this problem, but now that I’ve seen this research, it makes sense that there would be chemical reactions going on there that are causing the mental fatigue:

“Influential theories suggested that fatigue is a sort of illusion cooked up by the brain to make us stop whatever we are doing and turn to a more gratifying activity,” says Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France. “But our findings show that cognitive work results in a true functional alteration — accumulation of noxious substances — so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.”

Pessiglione and colleagues including first author of the study Antonius Wiehler wanted to understand what mental fatigue really is. While machines can compute continuously, the brain can’t. They wanted to find out why. They suspected the reason had to do with the need to recycle potentially toxic substances that arise from neural activity.

To look for evidence of this, they used magnetic resonance spectroscopy (MRS) to monitor brain chemistry over the course of a workday. They looked at two groups of people: those who needed to think hard and those who had relatively easier cognitive tasks.

They saw signs of fatigue, including reduced pupil dilation, only in the group doing hard work. Those in that group also showed in their choices a shift toward options proposing rewards at short delay with little effort. Critically, they also had higher levels of glutamate in synapses of the brain’s prefrontal cortex. Together with earlier evidence, the authors say it supports the notion that glutamate accumulation makes further activation of the prefrontal cortex more costly, such that cognitive control is more difficult after a mentally tough workday.

So, is there some way around this limitation of our brain’s ability to think hard?

“Not really, I’m afraid,” Pessiglione said. “I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

There may be other practical implications. For example, the researchers say, monitoring of prefrontal metabolites could help to detect severe mental fatigue. Such an ability may help adjust work agendas to avoid burnout. He also advises people to avoid making important decisions when they’re tired.

Since I’ve come to accept that my brain is “disordered”, I’ve had a much easier time turning a desire to act into actual action. It helps to be able to recognize what my brain is doing, and then shift to working out how to dismantle that mental block, rather than trying to just push my way through it.

But knowing that I can do that doesn’t mean the mental blocks aren’t there anymore. The problem is that my primary tool for detecting those mental blocks is the same lump of meat that’s experiencing them. When I’m in the middle of a problem, I often don’t notice what’s happening – all I have is the experience of my brain resisting me. I’ve also had to accept that that’s going to keep being an issue, at least until I can afford diagnosis and treatment. At the root of it, there are times when my brain has a “knot” like you might get in your neck or back. You can’t brain your way out of a brain problem.

From what I can tell, this research shows that that’s true for everyone some degree, but I’d guess that it’s more so for certain people under certain conditions. I think that the panoply of neurotypes and mental problems is largely due to the complexity and plasticity of the brain. Muscles can be trained, and there’s a great deal of flexibility and diversity when it comes to what you can train them to do. Brains seem to go way, way beyond that, so it’s frankly not shocking that they can turn out so many different ways of processing, interpreting, and interacting with the world.

My worst period of mental burnout was a few years back, and I did not notice it coming. I’d been working hard, but I was feeling good about working hard. I was getting good feedback, and respect from people, and I was feeling good about my life.

And then I started sleeping through all my alarms. I just couldn’t wake up when I wanted to. I wasn’t just late for work, I’d sleep through half the day, and wake up to realize I’d missed a meeting, even if I’d gone to sleep at a reasonable hour. My brain just stopped cooperating. It didn’t matter how many alarms I had, or where I put them – I could either shut them off without ever gaining full consciousness, or sleep through them.

I was lucky to be in a forgiving work environment when that happened. I know a lot of other jobs would have just fired me, even though it wasn’t under my control. As a rule, people only matter in our society for the work they can do for someone up the ladder. I think it’s very telling that despite that rule, so many put so much of their time and energy into making life better for other people.

This also underscores the degree to which our current political and economic system is both unjust, and unsustainable. It turns out that the culture of pushing people to work past their limit never went away. The kinds of jobs changed (in the U.S.) as manufacturing and resource extraction was increasingly done overseas, but the same fundamental demand – that you work as hard as you possibly can – has never gone anywhere. Ever hear the phrase “mind over matter”? Ever hear someone be told they’re just lazy, or they just don’t want the job badly enough?

We’ve created a world in which it’s expected for everyone to push at least some part of their body to the limit on a daily basis, and if we don’t – or if our limit manifests in the wrong way – we deserve all the bad that comes to us. There is no way that this system is ever going to lead to a society that values human life or flourishing. There is no way that this is the best that we can do as a species.

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