Why the marathon comparison?

Hernandez frames the results with a running analogy.

"If you imagine a person running a marathon where performance is speed, someone who already feels very tired at the beginning of the race might find it harder to ignore that fatigue to maintain their pace later in the race," Hernandez told Medical Xpress.

The same logic applies to going out too fast. "If a runner runs faster than their usual pace at the beginning of a race, fatigue could be harder to ignore later in the race because more energy was used up. We wondered whether the same pattern might emerge as people tackle mental tasks throughout the day for their job or other responsibilities—a kind of cognitive marathon."

The data bore out that intuition. "The pattern resembled what we might expect in a running marathon: both feeling more fatigued and performing faster than usual were associated with a harder time overriding fatigue later on," Hernandez said.

What can you actually do with this?

The findings are preliminary in an important way. They show associations between measurements taken hours apart in everyday life; they do not prove that fatigue or fast performance causes the later drop in fatigue override. The effects were also described as slight for the tiredness-performance link.

Still, the researchers see a practical possibility: people could read their own recent fatigue and performance the way runners read their pace, and schedule demanding work or breaks accordingly.

"Our results raise the possibility that, much as runners adjust their pace based on how they feel and how fast they are going, people could use their recent fatigue and performance to guide when they tackle demanding tasks or take breaks," Hernandez said.

He is careful about the limits of that idea. "Whether that approach helps people sustain cognitive performance during meaningful daily activities—such as participating in an important meeting or following complex medication instructions—needs to be tested."

What comes next?

Future studies could probe the observed patterns more deeply, for example by using a broader range of methods to measure both fatigue and processing speed rather than relying on self-reports and a single symbol-matching task.

Hernandez and his colleagues plan to keep exploring the factors that shape people's ability to maintain cognitive performance when tired. Their longer-term goal is to inform strategies that help people sustain mental performance during fatiguing days.

The study also benefited from an unusually collaborative setup. As Hernandez noted, "the principal investigators of the various studies came from a wide range of fields, but all shared an interest in fatigue and were even willing to help with the paper."

For now, the takeaway is modest but concrete: if you feel unusually tired, or you have been tearing through tasks unusually fast, the hours ahead may demand more effort to stay sharp. Pacing yourself, mentally as well as physically, may be worth considering.

Publication details: Raymond Hernandez et al, "Fatigue and processing speed predict subsequent fatigue override, evidence from a coordinated analysis of six EMA studies," Communications Psychology (2026). DOI: 10.1038/s44271-026-00537-1.