In her exploration of the epidemic of overtraining syndrome among ultrarunners a few years ago, Outside contributor Meaghen Brown told the story of Mike Wolfe, a North Face pro from Montana whose career had ground to a halt in the face of a debilitating but unexplained set of symptoms. Over the years, scientists have kicked around numerous possible theories about overtraining: unbalanced hormones, an immune system gone haywire, energy deficits, stress overload, and so on. But Wolfe was ready to consider a simpler possibility: “[A]t some point,” he mused, “the mind quits before the body and just says, ‘Enough.’”

There are some intriguing echoes of that idea in a recent study published by French researchers in Biology Letters—but, far from closing the book on overtraining, the results raise as many questions about the nature of extreme fatigue as they answer. The most telling conclusion is that the distinction between mental and physical fatigue, at least when it comes to prolonged tests of endurance, is thin to the point of vanishing.

The study involved 37 serious triathletes, half of whom increased their training load by 40 percent for a three-week period in order to induce a mild and reversible form of overtraining (what sports scientists call “overreaching”). At the end of this period, they completed a series of cognitive and decision-making tasks in a brain scanner, answering questions like “Would you prefer $10 now or $50 in six months?” The eye-catching result is that the overtrained triathletes became more likely to choose immediate rewards over delayed (and superior) gratification compared to the control group. This is the finding that led to headlines like CNN’s “Too much exercise could lead to bad decisions on what you eat and buy”—which is, to put it mildly, a bit of an extrapolation.

It’s worth stepping back for a moment to understand where this study comes from. The lead researchers, from a neuroimaging group studying motivation and behavior in Paris, published a previous study back in 2016 in which they observed essentially the same effect after subjects spent “more than 6 hours (an approximate workday)”—vive la France!—doing mentally fatiguing cognitive tests. Willpower, as the famous marshmallow test studies suggest, is a pretty useful trait in all sorts of life contexts. The researchers wanted to explore the popular idea that it’s a finite resource that gets exhausted if you use it too much. The problem with that idea is that (a) studies keep contradicting it, and (b) no one can figure out what actual resource, on a biological level, is being used up.

The 2016 study suggested that you need longer periods of time—hours rather than minutes—to measurably deplete your willpower (or, to put it more scientifically, cognitive control). And it also located a specific region of the brain whose activity decreased as the subjects became more impulsive: the middle frontal gyrus (MFG), a small subunit within the lateral prefrontal cortex that’s part of the brain’s cognitive control network. Choosing the delayed-gratification option seems to involve firing up the MFG, and for some reason that gets progressively harder if you tax the MFG over and over. It’s tempting to wave your hands and say that the MFG is getting “tired,” but other regions of the brain keep on firing without any trouble.

The overtraining study was suggested by researchers at France’s National Institute of Sport, Expertise, and Performance, who deal on a regular basis with athletes whose careers are derailed by overtraining. And perhaps the most remarkable thing about it is how similar its results are to the earlier cognitive one, with the same shift toward immediate gratification, and the same decrease in MFG activation. Ramping up your training by 40 percent for three weeks sounds absolutely brutal to anyone who has ever done serious endurance training. Of course your decision-making is impaired! But the same thing happens after the cognitive effort of a single simulated workday. It’s hard to imagine that the familiar thank-god-it’s-5 P.M. (or, in France, 3 P.M.) feeling is really what ended Mike Wolfe’s career.

There are two other surprising nuances. One is that the overtrained athletes didn’t perform any worse on tests that required cognitive control such as the N-back test (which involves, for example, being shown a series of letters and pressing a button whenever the current letter is the same as the one shown three letters ago). The athletes didn’t get dumber, and when there was an objective “correct” answer, they could still get it. It was only when there was a subjective choice (less money now or more later) that their preferences changed. The mind says, “enough.”

The other surprise was that acute exhaustion didn’t seem to produce the same effect. In the final testing protocol, the athletes did two sessions in the brain scanner, separated by a 45-minute all-out time trial on the bike. Despite pushing themselves to the brink of exhaustion, neither the overtrained nor the normally trained athletes showed any difference in their delayed gratification from before to after the time trial. These brain changes, then, aren’t simply a consequence of fatigue.

As I said at the top, we’re left with more questions than answers. This isn’t the hidden cause of overtraining finally unveiled. But it provides a nice mirror image to the work of endurance researcher Samuele Marcora, who showed that mental fatigue impairs physical performance. Now we see that the arrow goes both ways, and that bolsters the idea that mental and physical exertion both draw on the same finite well of... something. Exactly what that something is remains a mystery, and this new study likely tugs at just one thread among many. But for every endurance athlete who, while in the throes of heavy training, has sensed that life’s many marshmallow tests seem to be getting harder, the results are a nice validation: you’ve got a sprained MFG.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.