Energy used by the brain is quite severe, considering how little of our body is made up by grey matter. According to a report published in 2012, the brain weights only 2 per cent of the total body, yet requires 20 per cent of our daily energy needs. And even older research supports the theory that thinking harder burns more glucose.
A notable 2001 study out of Northumbria University had half of their test subjects complete complex verbal and maths problems, while the other half mindlessly pressed a key on repeat. Those who were completing the verbal tasks showed a significant drop in blood glucose levels, as a suggested result of greater energy expenditure.
“A period of intense cognitive processing leads to a measurable decrease in levels of peripherally measured blood glucose, which may be linked to increased neural energy expenditure,” says the research.
“If we were to put you in a scanner and we looked at what’s going on [in your brain] while in front of the TV or doing a crossword, your brain’s activity would change if we gave you a demanding task, and it would use more energy,” said Dr. Marcus Raichle from Washington University School of Medicine in St. Louis in Time’s investigation into the matter.
Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction