How can sitting there and thinking wear you out?

Hey Brainiacs, Julia here for DNews.

Have you ever been really tired after spending all day sitting around at school?

How can you be so exhausted from literally just sitting all day?

Why is learning so tiring?

Well, let's start with the body's energy usage.

About 30% of the calories you eat go towards maintaining your current muscle mass.

While our brain only makes up 2% of your body weight, it gobbles up 20% of the energy you take in.

One study published in Proceedings of the National Academy of Sciences USA found that 2/3s of the energy the brain uses goes to the firing of cells.

But the other third, is basically just brain maintenance.

So where does that energy come from?

Most of the energy the brain eats up is in the form of glucose from the things we eat.

When neurons in the brain fire, they need extra oxygen and glucose from nearby capillaries.

So scientists think that when performing a difficult mental task, the neurons would eat up more glucose.

And so there'd be less glucose in the bloodstream if you'd measure it after something like a tiring test.

And that is what some studies show.

Another study published in the journal Physiology Behavior found that there was less blood glucose in the bloodstream, which the researchers thought was linked to neurons firing on all cylinders.

Another more recent study published in Human Factors: The Journal of the Human Factors and Ergonomics Society found that people get more exhausted when exerting both physical and mental faculties than when just simply working out.

The researchers specifically noted that when performing both physical and mental tasks, there was strong activity in the prefrontal cortex and there was lower blood oxygen levels in the PFC after a combined physical and mental task than when just doing physical work.

So following that line of thought, eating foods high in glucose should improve mental performance.

One review published in the European Journal of Pharmacology found that glucose can improve memory, but it really depends on the dose and the person.

So there is something about mental tasks that makes our brains work a little harder and take up a little more glucose.

But why does it actually make us tired?

Well, results are mixed.

One small study published in the journal Psychosomatic Medicine found that people who just completed memorization and attention tests ate 200 more calories than those who just relaxed.

But it might be more than just brain drain.

One scientist, Claude Messier of the University of Ottawas, thinks maybe it's just stress, that those students were just stress eating, which is a very real thing.

So while test taking might be zapping your energy, maybe it's the classroom you're in.

Matt Lieberman explains how carbon dioxide in the classroom might make for slower brains.

It turns out even levels as low as 1,000 parts per million can reduce the brain's ability to focus and function properly.

And considering that we produce carbon dioxide every time we exhale, that's a big old problem.

So what was the last time you felt tired after a test?

Tell us about it down in the comments below.

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