Like your muscles, mental activity needs fuel to function

Our minds are often discussed as abstract ideas, but our brains is a hunk of biological material. While brains aren’t muscles, the three-pound collections of cells share a number of traits with our biceps and pectorals, in that they consume energy to do work, and in doing so can wear us out. Since your brain can’t exactly do lifts or curls though, it’s little trickier to measure exactly how mental efforts are consuming energy and leaving us tired all over.

We know that while on your average day, your body’s muscles account for 30 percent of your energy usage, your brain wants 20 percent of it. That’s a lot of energy for something that weighs a lot less than your total muscle mass, and two-thirds of that is thought to go towards mental processes of some kind (as opposed to cell maintenance.) One way all of this has been measured is in tracking how much fuel gets used, which just happens to be the same fuel that your muscles need work done— glucose and oxygen.

Drained by the brain

Tracking oxygen and glucose levels has helped us understand how much of a metabolic demand your brain puts on the rest of your body. People doing difficult mental exercises turn up with lower glucose levels in their blood afterwards, despite otherwise exerting very little physical effort to sit in a chair. Experiments have also found that people exercising their muscles use less glucose than people exercising and thinking hard at the same time.  In a less direct measurement, people who just finished an academic test were likely to want a high-calorie snack more than people who just sat in a chair, although it’s unclear if that was more of an emotional stress response than a desperate need to refuel.

So, does this mean that when mental work has wiped you out you should reach for a candy bar, er, some fruit to perk up, instead of some coffee or soda? Aside from the other health benefits of eating fruit instead of refined sugars, it is possible that the natural sugars in say, an apple, can get your glucose levels back up, making you feel less exhausted overall.

Source: Why Does Thinking Hard Make You Tired? by Jack J. Shamama & Julia Wilde, TestTube