The Real Reason You're So Tired

It’s 6:42 p.m. You come home from the office, and you’re simply exhausted. But I’ve been sitting at my desk all day, you think to yourself. It’s not like I’ve been out running a marathon. But in a way, you have been.

Researchers at Texas A&M Health Science Center School of Public Health have discovered that our brains are more like our biceps than we previously thought. Just as our muscles can get tuckered out, the brain can become overused and can suffer from fatigue as well. The findings came out of a study that looked at the interaction between physical activity and mental fatigue. For this study, physical activity was considered daily activities, rather than exerting yourself via exercise.

“Existing examinations of physical and mental fatigue has been limited to evaluating cardiovascular, muscular and biomechanical changes,” wrote the study’s coauthor, Ranjana Mehta, Ph.D., an assistant professor at A&M. “The purpose of this study was to use simultaneous monitoring of brain and muscle function to examine the impact on the PFC while comparing the changes in brain behavior with traditional measures of fatigue.”

The combination of mental tasks and physical tasks—let’s say you are typing and creating a Powerpoint presentation, for example—activates the prefrontal cortex (PFC) in the brain. After a lot of thinking, there will be lower blood oxygen levels in the PFC. This is what makes us feel even more fatigued than if we were solely doing a physical task, like chopping wood or jumping rope.

So the next time you’re thinking hard, give yourself occasional mental breaks. You don’t want to sprain that brain!