Why Thinking Makes Us Feel Tired

Have you ever wondered why you feel tired at the end of a workday, if all you have done is sit in front of a computer?

Numerous studies have shown that using our brain for challenging or stressful tasks drains energy and makes us tired.

This happens for multiple reasons.

Thinking increases glutamate.

A recently published study in Current Biology found that participants who spent more than six hours working on a mentally taxing assignment had higher levels of glutamate an important signaling molecule in the brain. Too much glutamate can disrupt brain function.

The scientists thought the effects of cognitive fatigue could be due to metabolic changes in the brain. They enrolled 40 participants and assigned 24 of them to perform a challenging task. For example, watching letters appear on a computer screen every 1.6 seconds and documenting when one matched a letter that had appeared three letters ago. The other 16 participants were asked to perform a similar, but easier task. Both teams worked for just over six hours, with two 10-minute breaks.

Using a technique called magnetic resonance spectroscopy, the researchers measured glutamate levels in the brain’s lateral prefrontal cortex. The prefrontal cortex is the home of cognitive control — the part of the brain that allows people to suppress their impulses. “If you get stung by an insect, you want to scratch,” said Antonius Wiehler of the Paris Brain Institute. “If you’re stopping this reflex, that would be cognitive control.” It’s also the system that humans rely on to choose tempting short-term rewards, such as an unhealthy snack, over long-term gains.

The researchers found that by the end of the day, participants who labored on the more difficult task had accumulated more glutamate in this region of the brain than those who had worked on the easier task. And, given a choice between an immediate cash reward and a larger reward that would come months later, they were more likely to choose the smaller, short-term reward than they were at the start of the day.

In addition to increased glutamate, our brains burn a lot of sugar when they are working hard.

Glucose (a form of sugar) is the primary energy source for every cell in the body. Because the brain is so rich in nerve cells, or neurons, it is the most energy-demanding organ, using one-half of all the body’s sugar energy. Brain function such as thinking, memory, and learning are closely linked to glucose levels and how efficiently the brain uses this fuel source. For example, if there’s insufficient glucose in the brain, neurotransmitters, the brain’s chemical messengers, are not produced and communication between neurons breaks down. Additionally, hypoglycemia, a common diabetic complication caused by low blood glucose levels, can lead to loss of energy for brain function. It’s also linked to poor attention and cognitive function.

So, there is a biological reason why I crave something sweet every afternoon! Think of all the “fuel” craved when cramming for a test or working late on a project. Our brains are demanding fuel to keep powering through!

Understanding the science behind our behavior can help us make healthier choices. Try to forego junk food and fuel up with healthier sugar sources like fruit instead. Take regular breaks to clear your head. A few minor changes can make a big difference in your energy levels, cognitive function, and overall health.