Mental Fatigue

Intense and chronic fatigue is a symptom of many different conditions, including myalgic encephalomyelitis (often called chronic fatigue syndrome), PTSD (post-traumatic stress disorder), multiple sclerosis, Parkinson’s disease, and various types of depression and anxiety disorders.

It’s also the primary symptom of what’s typically called “long Covid,” a collection of health issues that can develop and persist after a bout of Covid-19, and which also includes symptoms like breathing problems, sleep disorders, and memory issues.

The prevalence of long Covid (around six in every 100 people who catch Covid-19 will suffer persistent symptoms for three months or longer after the infection itself has subsided) has sparked renewed interest in figuring out what exactly causes the mental fatigue associated with this and so many other health issues and conditions.

This type of fatigue seems to manifest naturally, in everyone, after long periods of intense focus—the sort of focus that involves the conscious regulation and direction of thought or the maintenance of one’s attention on a given task for stretches of time.

There are many ways this sort of cognitive effort might lead to cloudy, uncontrolled, unfocused thinking, but it’s generally suspected this fatigue is a protective mechanism that allows our brain to tell us, hey, slow down, I’m running out of resources, or possibly hey, chill out, I’m accumulating too many of the toxins that result from this kind of labor. If that’s the case, cloudy thinking might be similar to pain in that it flags something bad that’s happening in order to keep us from doing more of that thing, helping us avoid further harm.

That depletion of resources might then make it difficult for us to continue performing cognitive tasks that use said resources (like glucose or lactate) as fuel, or it could be that the buildup of protein fragments or neurochemical messengers like glutamate or adenosine might disrupt communication between our synapses and/or increase neuroinflammation.

A recent piece published in Nature assesses some of the studies currently being run by researchers trying to determine, first, what causes mental fatigue, second, whether it’s connected to physical fatigue, and third, how this type of fatigue is connected to the way we behave—including the decisions we make, whether and to what degree we’re willing to delay gratification for long-term benefit, and how our preferences change when we’re fatigued versus when we’re not.

At the moment, many of these studies are predicated on correlative effects and the understanding, for instance, that while we can see the buildup of things in the brain, can see inflammation and note the depletion of resources when different sorts of cognitive tasks are performed, we can’t say for certain what causes what. We can see correlation, but not definitively identify causation.

Ultimately, the hope is that all of this research will help us better understand all of these conditions and issues, from Parkinson’s to long Covid, and that it will point to treatments that might help people who suffer mental (and possibly physical) fatigue.

That said, most involved researchers are quick to note there’s unlikely to ever be a one-size-fits-all solution to mental fatigue, especially in cases where getting more sleep and committing to a regular circadian rhythm (and proper exposure to the natural light that reinforces it) haven’t been effective.