How do we determine whether or not a particular piece of work is worth the effort? It’s natural to feel a bit more or less motivation than usual while doing tasks on certain days. Interestingly, researchers from the Universities of Birmingham and Oxford report that motivation is largely dependent on how our brains are processing fatigue in that moment.

Feeling fatigue after a long day’s work is a common state for many people. When we’re tired, our motivation is usually at its low point. Nothing matters more than getting some rest. While modern science has discovered the brain mechanisms in play while people decide whether something is worth the effort the task requires, the role fatigue plays in that process has remained a mystery.

Different Types of Fatigue Affect the Brain Differently

In an effort to answer that lingering question, the team discovered that fatigue indeed has a detrimental effect on motivation. While experiencing fatigue, the study finds individuals are less likely to work or put in real effort, regardless of the reward.

Of course, that finding in and of itself isn’t very surprising. Notably, however, study authors also report the discovery of two distinct types of fatigue detected in different brain areas. One variety of fatigue people experience has a short-term feeling which usually subsides after taking a short rest. The second type of fatigue takes longer to build up, kills motivation, and doesn’t go away after a quick break.

“We found that people’s willingness to exert effort fluctuated moment by moment, but gradually declined as they repeated a task over time,” says Oxford’s Tanja Müller, first author of the study, in a university release. “Such changes in the motivation to work seem to be related to fatigue – and sometimes make us decide not to persist.”

Researchers asked a total of 36 young, healthy participants to complete a computer task in which they had to exert physical effort in order to win money. Each person completed over 200 rounds and each time they were asked if they wanted to “work” (squeeze a grip force gadget) and earn more or rest and earn less.

From there, scientists constructed a mathematical model to determine how much fatigue a person may be feeling at any point in the experiment. The model also helped to figure out how much that fatigue may dictate their work or rest decisions. Meanwhile, each participant underwent an MRI scan while they worked on the task. This made it possible for researchers to compare the mathematical model’s forecasts with real-time brain activity readings.

What’s Going on in the Brain When You’re Tired?

MRIs revealed areas of the brain’s frontal cortex indeed displayed activity that fluctuated in agreement with the model’s predictions. Simultaneously, a different brain area called the ventral striatum signaled just how much fatigue was influencing participants’ motivation to keep working.

“This work provides new ways of studying and understanding fatigue, its effects on the brain, and on why it can change some people’s motivation more than others” concludes Dr. Matthew Apps, senior author of the study from the University of Birmingham’s Centre for Human Brain Health. “This helps begin to get to grips with something that affects many patients lives, as well as people while at work, school, and even elite athletes.”