Brain drain: scientists look at why mental exertion triggers exhaustion

It feels the same on a Friday night. You agree with your friends on where to go for a night out after a long day of work.

By the time you know what to wear and where you left your keys, a night on the sofa is more appealing than one on the tiles.

Scientists think they may be able to explain why you are so tired before you get to the bus stop.

There is a chance that the brain could suffer from the same problem as the muscles. Hard mental yards and resisting the temptation to give up throughout the day are both taxing.

The accumulation of a potentially toxic neurotransmitter in the prefrontal cortex is caused by long-term mental activity. The brains activity is suggested to be slowed to manage the build up.

The first author of the study said that the brain exerts cognitive control even when it resists scratching an itch. He said that fatigue can be caused by repeated demands on cognitive control functions.

The prefrontal cortex is the region of decision-making and cognitive control that is used when the brain ignores an impulse.

The brain chemistry of 40 people was monitored by the team. They formed two groups that did both hard and easy tasks.

Researchers measured the levels of a neurotransmitter in the brain. The harder the tasks, the higher the amount of glutamate in participants.

A lot of thinking requires the brain to resist the urge to do less. The brain chemistry behind it has remained unclear.

According to researchers, high levels of cognitive control can cause the brain to accumulate a substance that can be harmful.

The region of the brain that controls the tasks we set participants has been found to have a build up of glutamate. The brain has a mechanism that may slow down activity.

The researchers theorize that mental fatigue could be related to the build up of glutamate in the brain. It's likely that the accumulated glutamate is happening during sleep.

The groups performing hard and easy tasks recorded the same level of fatigue when they were asked to report their fatigue levels. Those doing the easy task were unaware of the difficulty of the other, according to researchers.

The Institute of Neurology at University College London, who was not involved in the study, said that the fact that glutamate levels don't track reported fatigue is disappointing, but not surprising.

The researchers thought that other substances could be linked to fatigue. The study measures a single neurotransmitter in a specific part of the brain, so we have to look at it more global.

She said the results were positive. Muscle fatigue is caused by the build-up of lactate in the muscles. This is the first proof that something similar happens in the brain.