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When we engage our brains in a mentally taxing task, we eventually reach a point where we hit a wall or run on empty. Even when the rewards of finishing are obvious, we often find ourselves simply stopping. Exactly why this happens has long remained a mystery. We can easily point to tiredness or lack of motivation, but what is driving this disengagement at a biological level?

That is precisely what a team of scientists led by Hongmei Xia at Ludwig-Maximilians-Universität München in Germany set out to answer in experiments involving 31 healthy volunteers.

Each participant took part in a demanding 20-minute memory test on two separate days. During one session, they received electrical stimulation designed to increase activity in part of their prefrontal cortex. During the other, they received a fake version that delivered a brief initial tingle to make it feel like the real thing.

The test was designed to induce mental fatigue. To measure its effects, researchers asked participants, both before and after the test, whether they would be willing to complete additional memory tasks. They could choose between easier ones for smaller cash rewards and harder ones for more money.

The research team also measured brain chemistry before and after the tests using magnetic resonance spectroscopy. They were paying particular attention to glutamate because previous research has linked this brain chemical to mental exhaustion.

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