Understanding Brain Energy Consumption

You often hear about the 2/20 split — that the brain comprises 2% of our body weight yet uses 20% of our energy. It’s fascinating, and it kind of leads many to believe that “thinking” is super taxing.

However, a recent study adds some complexity to that understanding. A team led by neuroscientist Sharna Jamadar from Monash University found that performing effortful, goal-oriented tasks only results in about a five percent increase in energy use compared to just resting. So, when you’re tackling a problem, you might not be consuming much more energy than when you’re at ease.

So, what’s happening in that brain of yours? Mostly, it’s just keeping everything running smoothly. There might not be any visible activity, but the brain is busy processing information, maintaining electrical activity, and ensuring its networks are ready to react. As highlighted by BrainFacts.org, the fundamental task of processing and sharing information through electrical signals demands a lot of energy.

This energy consumption doesn’t just go away when we stop actively working. Even at rest, our brains are still very much engaged and demanding energy.

Now, this brings me to something I’ve noticed about my own work habits, especially in those afternoon slumps.

For me, about three hours of writing in the morning feels like the limit. After that, I might still be staring at my screen, but I typically need to switch gears: editing, answering emails, or tying up loose ends. When I hit that wall, my instinct is often to see it as a fuel shortage. More caffeine, more pressure, just keep at it until I force the words out.

But maybe that approach isn’t the best. As Zahid Padamsey points out, mental fatigue isn’t about running on empty; it’s more about a built-in limit to energy expenditure. He suggests that when you’re fatigued, your body sets off mechanisms to conserve energy.

This aligns with my experiences. Instead of pushing through the fatigue, the smarter move is often to pause, step back, and let those quieter aspects of the mind come into play.

Sometimes, I’ll walk to a different café. Or, I’ll just put my phone away for a bit. Giving myself twenty minutes to think without pressure — you know, that kind of meandering thought that isn’t exactly work — surprisingly often leads to breakthroughs just as I’m ordering coffee.

What resonated most in this research was Jamadar’s observation that we previously dismissed ongoing background activity as mere noise. Now, it seems there’s actually quite a bit of meaningful processing happening in that noise.

Many of us have been taught to view thinking as productive, while idleness seems like a lack of real work. But the reality is quite different. Most brain activity happens continuously, whether engaged or not. That extra effort feels significant, but it’s just a minor addition to what our brain is already doing. Often, the deeper, quieter processing is where real work takes place — and moments of “rest” where nothing seems to be happening might just be when those crucial insights emerge.

I’m not a neuroscientist, and I approach this as a general reader of studies rather than an expert. Still, one practical insight I’ve gained is valuable: the hours I spend not actively working aren’t wasted. My brain is engaged in a lot of the same essential tasks, even in those moments spent staring out of the window. Sure, there’s a difference between writing and daydreaming, but in terms of energy usage, that distinction might not be as large as it feels.