We’re all familiar with the sensation of mental fatigue. Your mind feels sluggish and bleary, like it’s gone soft around the edges. You also feel a little cranky and sleepy, and making decisions, even minor ones, is a struggle.
The longer you go without sleep and food, the more likely you are to experience this sort of mental lethargy. No surprise there. But comb through the research on cognitive fatigue and you’ll find a mountain of studies that suggest there are certain mental tasks that seem to drain our brains faster than others.
“Multitasking is a myth.”
Some of the most robust evidence suggests that multitasking is particularly fatiguing. A 2009 study in Brain Research found that people who spent two hours engaged in a multitasking challenge gradually made more errors and had slower reaction times, which the authors attributed to mental tiredness.
“Multitasking is a myth,” says Daniel Levitin, professor emeritus of psychology at Canada’s McGill University and author of The Organized Mind: Thinking Straight in the Age of Information Overload. According to Levitin, the human brain can concentrate on only one thing at a time, so when you ask your mind to manage several cognitive tasks at once, “all the switching is neurobiologically expensive and will tire you out.”
Making decisions is another chore the brain finds especially taxing, Levitin says, which may help explain why sorting through your inbox can be such a slog. He reels off all the decisions involved in checking email: “Do I read this now or later? Do I respond now or later? Do I forward this to someone? Do I need to get more information before I can answer?” If your day involves heavy doses of both multitasking and decision-making, it’s no wonder you feel mentally exhausted by the end of it.
But here’s the thing: Other researchers argue that mental fatigue is not task-dependent. That’s not to say your brain doesn’t get worn out; it does. But according to this view, multitasking and decision-making aren’t inherently more draining than any other mental chores.
“We’ve tried to get evidence for the differential depletion of cognitive reserves across different cognitive tasks, and we got no evidence at all,” says Ewan McNay, an associate professor of behavioral neuroscience at the University at Albany. “There is definitive evidence that harder tasks cause the brain to consume more fuel” — and by “fuel,” McNay means glucose, which is the brain’s main source of energy — “but there’s no evidence that one kind of task leaves you feeling more mentally tired than any other.”
Last year, McNay published a review paper on the topic of mental fatigue and self-control. For years, an influential body of research has argued that self-control (aka willpower) is a limited resource. But McNay and his colleagues concluded there’s no evidence that exerting your willpower induces mental fatigue more so than other cognitive tasks.
There’s plenty of evidence to support his take. Returning to that 2009 Brain Research study, those experimenters found that even after two hours spent on an arduous task-switching activity, participants were able to improve when they were offered money as a reward. In other words, their drop-off in performance was due to a lack of motivation — not brain exhaustion.
“I suspect that much mental fatigue is psychologically driven rather than biologically driven,” McNay says. How much brain fatigue you feel may depend in part on how much you enjoy what you’re doing. “I’m not a good piano player, but I enjoy it,” he says. “If I spend an hour trying to play piano, I can feel that I’ve been thinking hard, but it doesn’t make me feel tired in the same way as doing something I don’t enjoy.”
Another way to look at mental burnout is through the lens of motivation and reward. “If you’re doing an activity that’s intrinsically valuable to you, you won’t feel as much fatigue,” says Glenn Wylie, director of the Rocco Ortenzio Neuroimaging Center at Kessler Foundation.
Wylie says there’s evidence that several brain regions become more active when a person feels cognitively spent. And his research has shown that some of these same regions light up when your brain realizes it’s making mistakes. “I interpret fatigue as a signal the brain generates to tell itself that it’s had enough — that it’s time to stop doing an activity and move on,” he explains.
He says moving on from the task at hand can help dispel this perception of fatigue. “It’s refreshing to stop doing something and take some time to do something else,” Wylie says. The trick is to make sure the “something else” isn’t just more of the same from your brain’s point of view.
For example, taking a break from drafting emails to scroll your social feeds may seem like a big change, but these screen-based tasks are engaging many of the same brain areas and functions, Wylie says. While it’s true that everything in your head is connected, he says going for a walk or listening to music are the kinds of activities that should give your mind a change of pace from typical office work. Eating, sleeping, exercising, and meditating are also tasks that can help shoo away your sensation of mental fatigue.
Again, much of this is debatable. “The definition of cognitive fatigue and why we perceive it and whether it’s a biologically based thing are all very controversial,” McNay says. “What makes your brain fatigued? The act of thinking.”