[nebulous_two]

It might be worth thinking about what it is that is being depleted, exactly. When I "run out of willpower" it doesn't really feel the same as when I "simply cannot do it anymore". If I lift a weight enough times, eventually I simply can't anymore, no matter how much I will it. It's not a decision, like a decision to stop working on a problem. That would be a lack of willpower to continue...?

Is there really a mental equivalent to physical exhaustion that leaves us beyond the ability to make a decision? Is that what running out of willpower would be?

[Metalbourne]

Believe it or not, even something like physical exertion has hard-to-define limitations. The amount of reps that you can do of an exercise is more based on how forcefully your brain drives your nerves to activate your muscles and keep going. If you have a habit of sticking to sets of 10 reps, odds are you will feel exhausted at 10 reps, and this is because once you hit your magic goal, you're no longer applying the same concentrated mental energy, and you suddenly feel tired and stop there. But if you did that set like as if it were the last set of your life, or like you were at the olympics trying to break records, you'd be able to push 15 or 20 reps, rather than just the 10 that you do as your comfortable limit. You have the physical ability to keep doing something until the moment that your muscles lock up from lactic acid buildup and you just drop. But people rarely ever reach that state. They stop much sooner because pushing further requires more concentrated brain input which they don't want to dedicate. Maintaining your current routine is effortless, and we tend to favor the easy, comfortable. Pushing your limits is uncomfortable, and in a world where we have become so accustomed to prioritizing indulgence and comfort it becomes hard to break out of our safe zones.

[nebulous_two]

Reading your comment made me realize that you're right, there really isn't such a hard rule even with physical exertion. Even putting some motivating music on might make you push for an extra rep or two. If a gun was to your head maybe you'd do even more. The extreme end might be phenomena like "dead man's grip" where inhuman strength is shown while on death's door.