There’s a certain kind of bleh feeling that arrives after a mentally taxing day. It’s not quite the same thing as being physically tired (though it could similarly leave you ready to flop down on the couch and do nothing). More specifically, it makes using your brain annoyingly difficult, even if it’s just to decide on what to eat for dinner. Scientists call this phenomenon cognitive fatigue, and according to new research, there may be a real physiological basis for why it happens and how it affects our decision-making.

To pinpoint the uniquely tiring effect of difficult work—as opposed to less cognitively demanding stuff—researchers split 40 participants into two groups, and had the first group do challenging mental exercises (call this the “hard” group) and the second do simpler versions of these exercises (call this the “easy” group) for six hours with two 10-minute breaks. “The idea was to keep these exercises as similar as possible, with the only difference being how much focus was required, so we could control for things like boredom in either group,” says neuroscientist Antonius Wiehler, PhD, lead researcher on the study.

As an example, the hard group was asked to look at letters in random sequence on a screen, shown one after the other every 1.5 seconds, and for each letter, indicate whether it was the same as the letter that appeared three letters before it. Whereas, the easy group was just asked to look at letters and note whether each letter matched the one just before it.

To search for proof of cognitive fatigue at play, the researchers then had all the participants make choices with real effects on their lives. Each decision included one low-effort, quick option (like getting a small sum of cash immediately) and one more effortful, long-term option (like waiting to get paid a larger sum of cash by bank transfer in a couple weeks).

“It was as if the ability [for participants] to resist the tempting immediate-cash option had become more costly or more difficult after all their hard work.” —Antonius Wiehler, PhD, neuroscientist

“What we found is that the hard group, or the group that had to do the mentally difficult work all day, more and more preferred the immediate option,” says Dr. Wiehler. “It was as if, somehow, the ability to control themselves or to resist the tempting immediate-cash option had become more costly or more difficult after all their hard work.”

The same association held true with other choices, too. For example, when participants were offered money based on the amount of time they chose to pedal on a stationary bike, the people in the “hard” group consistently opted for less money in exchange for biking less, when compared to their peers from the “easy” group. In this way, cognitive fatigue likely affects decision-making by increasing the appeal of low-effort, immediately gratifying choices. But it was only when the researchers analyzed the brain scans of the participants that they gained insight into how and why that happens.

How cognitive fatigue may change your decision-making, often without you even noticing

While both groups of participants were chugging through their work, the researchers also conducted brain scans using magnetic resonance spectroscopy (think of it as a souped-up MRI), which allowed them to track biochemical changes and learn how cognitive fatigue may affect the biological process behind decision-making. In particular, they were looking for buildup of a potentially toxic byproduct of brain activity called glutamate in the prefrontal cortex—the region of the brain that’s responsible for cognitive work. What they found: People in the “hard” group showed greater accumulation of glutamate in their prefrontal cortexes than their “easy” group counterparts.

“This glutamate buildup could be the reason why activity gets down-regulated in the prefrontal cortex after a long day of work,” says Dr. Wiehler. Essentially, all the mental effort sends your neurons firing at max capacity, generating lots of glutamate—and after a certain point, it reaches too high a level for cognition to flow properly. Cue: cognitive fatigue. In that condition, it may be physiologically tougher to make choices or exert any cognitive control, which is why Dr. Wiehler suspects that the participants from the “hard” group started acting more on impulse and making low-effort, easy-reward choices when given the opportunity.

The tricky thing is that these participants likely wouldn’t have been aware of how cognitive fatigue was affecting their decisions if you asked them, says Dr. Wiehler. “When you’re experiencing cognitive fatigue and someone asks you to make a decision, you might say, ‘Oh, I prefer this’ [easy-reward choice], and you’re convinced that this is your preference, but if you weren’t fatigued, you would likely have a different preference.” Which is all to say, pushing through cognitive fatigue could lead your decisions to change without you even knowing it. 

In real life, that might show up as a tendency to choose any lower-lift, more instantly gratifying, or impulsive choice after a long work day—as in, ordering a burger for dinner, rather than making a salad. It’s also likely that you’ll invest less effort into whatever follows a mentally strenuous workday, perhaps shortening a workout or taking the easy road out of a conversation, as your brain struggles to function fully amidst the glutamate overload.

How to manage cognitive fatigue in your day-to-day life

Do enough mentally taxing work of any kind, and eventually, cognitive fatigue will set in, bringing along its downstream effects on decision-making. Instead of seeking to avoid it, consider it a natural check on your workload. Physiologically, you actually need to take a break after a certain point to make sure your brain can keep working well.

As for how long that break needs to be in order to rid the brain of extra glutamate? That isn’t fully clear, says Dr. Wiehler. In the study, he found that the 10-minute breaks didn’t seem to have any restorative impact on participants, meaning the brain likely needs longer than that to recover, in any significant way, from cognitive fatigue.

What the researchers do know is that glutamate excess is wiped out by a night of sleep, leaving you, literally, clearer-headed in the morning. Consider that yet another reason to prioritize good sleep, particularly around a time when you’ll be making any big decisions.

In that vein, it’s also smart to make important decisions in the morning, rather than at the end of a workday, to ensure that you’re actually making them with your best interests in mind, says Dr. Wiehler. You might also pre-plan certain nighttime decisions in the morning—like what you’re going to eat for dinner or do after work—and then just execute on them once the time rolls around in order to avoid the decision-making slump.

During the day, Dr. Wiehler suggests alternating between cognitively taxing work and easy or routine work, if you can, so that you’re not doing a ton of hard work at once and swiftly overloading your brain with glutamate to boot. Though this wasn’t a part of the study, he suspects that the intervals of less focus-demanding work may give the prefrontal cortex a bit of a break and help keep it from fatiguing as readily.