Tired brain!

By Arvind M. Dhople, Ph.D., Professor Emeritus, Florida Tech

It’s not surprise that hard physical labor wears you out, but what about hard mental labor?  Sitting around thinking hard for hours makes you feel worn out, too.  Even get home from the office and feel exhausted, even though you’ve sat at the desk all day?  Scientists think they have figured out the reason why thinking hard can make you tired, giving new meaning to a “mental vacation”.  Researchers have new evidence to explain why this is, and, based on their findings, the reason you feel mentally exhausted (as opposed to drowsy) from intense thinking isn’t all in your head.

                A desire to curl up on the couch after a day spent toiling at the computer could be a physiological response to mentally demanding work, according to a new study that links mental fatigue to changes in brain metabolism.  The study found that participants who spent more than six hours working on a tedious and mentally taxing assignment had higher levels of glutamate – an important neurotransmitter signaling molecule in the brain.  Too much glutamate can disrupt brain function, and a rest period could allow the brain to restore proper regulation of the molecule.  At the end of their work day, these study participants were also more likely than those who had performed easier tasks to opt for short-time, easily won financial rewards of lesser value than larger rewards that come after a longer wait or involve more efforts. 

                Glutamate buildup doesn’t just muck up your brain, it affects your ability to make decisions as you become more and more fatigued.  The neuroscientists previously thought that mental fatigue was an illusion created by the brain to encourage a person to switch tasks when the one at hand became boring or repetitive.  But now it has been found that fatigue goes deeper, instead acting as a stop-gap before too many noxious chemicals can build up.  Fatigue could indeed be a signal that makes them stop working, but for a different purpose: To preserve the integrity of brain functioning.

                The dissociation is common in everyday life: for instance, when people go on working or driving and start making errors because they failed to detect their true fatigue state.  And of course, deep thinking can take place in any situation, not just during the workday.  Take the example of professional chess players: Even they make mistakes, “typically after 4-5 hours in the game that they would not make when well resting.

                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.  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.

                So, the scientists are hoping to use this system to learn more about how to recover from mental exhausting.  It would be great to find out more about how glutamate levels are restored.  Is sleep helpful?  How long to breaks need to be to have a positive effects?  Studies of cognitive fatigue could also be key to understanding how workers react to – and recover from – high-stakes mental work such as air-traffic control, in which even a brief loss of focus can cost lives.

                So, the scientists are going to find what does glutamate buildup means?  The neurotransmitter needs to be present at low levels for the brain to function properly, but when it’s overproduced or released in the wrong areas, it could be a toxic by-product.  And unfortunately, there’s no easy hack to reversing the potentially toxic effects of a hard day’s work.  So, I would employ good old recipes: rest and sleep!