There are two reasons for mental fatigue as best as is known/we can infer. The first is stress from the task (by which I mean the immediate thing that is correlated with cortisol levels). The second is that maintaining attention on a task, where there is not much reward or feedback on the task's progress makes maintaining attention harder and harder to do as time progresses. Imagine a very heavy door that wants to swing back shut the longer you hold it open. Resisting this too is linked with stress inducing mechanisms.

A good theory should explain why watching a television serial, a very complex task involving long term memory retrieval, short term memory of details, inferring various motivations, theory of mind (on a meta level too via genre savviness), agent modeling and prediction, language, visual, audio is less taxing than solving 1000 simple arithmetic problems. The resource management and attention based theories have better explanations for why that may be.

I find I can be highly productive with a highly inconsistent sleep schedule as long as I get significant amounts of sleep every few days. I don't think a consistent schedule is a requirement.

I heard different story: there is no such thing as "mental fatigue". The reason why we get tired when doing office work is:

- our muscles need to support our body while sitting. - most mental work includes some dose of stress, which is a real reason of fatigue.

Buy yourself best chair you can find plus do stress-free, enjoyable work and you can go 16 hours straight, without fatigue. At least that's how I felt about all-weekend-long Quake matches back then when I was living in dormitory.

I think it depends on the task. Playing Quake doesn't require significant amounts of creativity or learning, in the same way that programming or learning a new language would. We know that the brain needs sleep in order to process new memories, so that may be one factor in mental fatigue.

This is anecdotal but I have often noticed that I become more thoroughly fatigued and thus need more sleep when working on really hard programming problems vs less mentally taxing tasks. This implies that it's more than a physical phenomenon.

Anecdotally, more difficult mental work causes me to mentally fatigue faster which feels like good evidence against the claim

Dude i get fatigued by games, u have a point, but there is just less fatigue not 0 fatigue.

Why does your brain need to rest? What exactly 'builds up' and why don't regular tasks like the visual perception of the world cause this? And so on.

More specifically, mental fatigue is a result of dissonance. Working on your dissonance builds a strong and quiet mind.