How Being Mentally Tired Changes the Choices You Make

Have you ever felt so mentally drained after a long day that even small tasks felt too hard? Scientists at Johns Hopkins University and the Kennedy Krieger Institute wanted to find out why that happens. They studied how a tired brain makes decisions, and what they found could help explain everyday moments — like not wanting to cook dinner after a busy day. Their research was done by watching brain activity while people made choices about whether to do easy or hard tasks for money.

The feeling of being mentally worn out is called cognitive fatigue. It's not the same as being physically tired from running or lifting things. Instead, it's the kind of tired you feel after hours of thinking hard, making decisions, or solving problems. A nurse who finishes a long shift might skip a visit to one more patient. An office worker might feel too drained to hang out with friends after staring at a computer all day.

To study this, researchers watched the brains of 28 people using a special machine called an fMRI, which shows which parts of the brain are active. First, the participants did tasks that tested their working memory — the part of memory that lets you hold several ideas in your mind at once. They had to remember strings of letters, with some rounds being harder than others. The goal was to get their brains warmed up and a little worn out before the real test began.

After those tasks, each person was given a choice. They could take an easy memory test and win $1, or take a harder memory test and win up to $8. As people got more tired, they kept picking the easy $1 option — even though the harder test paid much more. This showed scientists that mental fatigue can change what feels 'worth it' to a person.

The researchers found that two areas of the brain were talking to each other more and more as people got worn out. One area, called the dorsolateral prefrontal cortex, is near the front of the brain and helps with decision-making. The other, called the right anterior insula, keeps track of how the body is feeling. As people got more fatigued, these two areas sent stronger signals to each other, making the person less likely to choose the harder task.

This research also matters for people who deal with stronger forms of mental fatigue. Conditions like depression, long COVID, and multiple sclerosis can make cognitive fatigue much more intense. Scientists hope that by learning which parts of the brain cause this fatigue, they can one day create medicines or therapies that help people who struggle with it every day.

The good news is that there are ways to push through mental tiredness when you need to. Research shows that if you find something interesting or understand why it matters, you're more likely to do it even when tired. Thinking about a task differently — like reminding yourself why it's important — can give you the boost you need to get it done.