Tired After a Long Day of Thinking Hard? Here’s Why

Sept. 16, 2022 – You’ve been doing deep focus work all day. Now you’re mentally fried. Wiped out. Exhausted. But you’re trying to wrap up a project. Should you power through?

New science has the answer: No, you should not.

In a Current Biology study, French researchers found that doing mentally hard tasks for more than 6 hours leads to a buildup in the brain’s prefrontal cortex of glutamate, a molecule involved in learning and memory, that can be toxic in high levels.

“Fatigue might be an adaptation to reduce the accumulation of glutamate,” says study author Antonius Wiehler, PhD, a researcher at the Paris Brain Institute. In other words, that tired feeling could be your brain’s way of telling you to stop so your glutamate levels won’t get any higher.

The researchers divided 40 people into two groups. One group spent more than 6 hours on mentally draining assignments, while the other was given easier tasks to do.

At the end of the day, the group that had to think hard showed more signs of fatigue, including reduced pupil dilation (linked to lower levels of effort, Wiehler explains) and a tendency to favor fast rewards and less effort.

For example, they chose to receive a smaller amount of money right away versus a larger amount later. And they were more likely than the other group to choose a lower difficulty level for a 30-minute task, and a lower resistance level for a 30-minute ride on a stationary bike.

In other words, they made choices that called for less self-control and therefore less effort.

“It must have become more costly for them to apply control,” says Wiehler.

Using magnetic resonance spectroscopy, the researchers also monitored the brain chemistry of the people studied, spotting the higher glutamate levels in the hard thinkers.

“It is important to limit glutamate release,” Wiehler says, explaining that’s because glutamate is a useful resource inside cells, but potentially toxic in excess outside or between cells.

How Can You Restore Brain Function?

One takeaway from this research: You are not a machine. You need rest to restore your brain after a mentally tough day.

“Breaks and sleep are important,” Wiehler says. So, make sure you’re taking 10- to 15-minute breaks throughout the day and getting that solid 8 hours of shut-eye at night.

And try to make important decisions when you’re rested, he suggests.

You might consider planning meals ahead of time to avoid eating unhealthy food after a hard day, or you can try exercising earlier so you can bring more effort to your workout.

Still, Wiehler notes that more research is needed to show that these tips can help.

“We’ll ask the questions: How is [glutamate level] restored during sleep? How long does [sleep] have to be? How long should breaks be?”