For years, researchers believed that mental fatigue was willpower running out, known as “ego depletion”. Although this explanation seemed logical, further testing largely disproved it. In reality, you feel fatigued because your brain constantly asks, “is this still the best thing I could be doing?”
Physical fatigue is easy: when you run, your muscles burn through fuel and build up metabolic by-products. As these accumulate and energy stores run low, the muscle fibers can’t contract as effectively, and they start to feel heavy and weak.
But with the brain it’s weirder: there’s nothing obviously getting exercised, but after a few hours of deep work you still feel cooked. So there must be something that won’t let me work for 12 hours straight, day after day, while keeping errors low and cognition sharp.
A more convincing model of mental fatigue is the opportunity cost: your brain constantly compares the current task reward vs the rewards of other things you could be doing. Working one more hour on the same task at 10 a.m. feels fine. By 3 p.m., it looks less rewarding than watching Netflix on the couch. That’s why you’re tired for deep work, but not for binging Stranger Things.
This is a huge step forward over ego depletion. You are not depleted, so you can keep going by adjusting the cost-benefit balance. When you first feel “I’m tired of this”, it is often about boredom, competing rewards and the current task’s low immediate reward, rather than a limit.
In this view, fatigue means “my brain thinks this task is no longer the best deal”. If mental fatigue is relative like that, you can tackle it from three angles:
Boost the value of the current task: add tiny rewards (snack, walk, chat) at the end of a work sprint; add social accountability (coworking, daily check-ins, post progress); write why this task matters to you.
Change the value of alternatives: phone in another room increases the cost of switching; full-screen apps remove the constant temptation to switch; make alternatives hard to start.
Change the time horizon: make tasks shorter; rotate tasks to reduce monotony, variance increases value; short breaks lower the current task boredom.
Does this mean you can grind for 12 hours straight? No. Exhaustion, sleep debt, and emotions still win. Ignoring them long enough will inevitably lead to burnout.
What this does give you is a way out of that weird limbo where you have time, energy, a clear goal... and still somehow end up doomscrolling. When you have the energy and the environment is right, you can tap into this framework to progress your goals.