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Mental fatigue is something that can manifest in different ways.

To keep it short, for me, It is like I can think down a path, but slowly, it is like I have this plodding speed, if I try to think 'quicker' (or more reactive/agile) it feels like a lot of effort, like I have to focus and push myself. The more effort I apply the more energy I use. The more energy I use the longer this state lasts for. The longer this state lasts for the more chance I develop physical issues. When I am in this state, I can't mentally fit pieces together. It is like I am wearing oven mits and trying to build lego. It just doesn't fit together. oh and I get really clumsy, my movement becomes really uncoordinated.

So it is like I have a smaller pool of energy, and I can spend it slowly over a longer period. Or faster over a shorter period. When I go over my limits, then see above.

The only cure, is rest, and that is usually about 3 days of not pushing myself mentally too hard, to get back to a reasonable baseline. It is improving, if we had had this conversation three years ago...

I have seen this in other devs, a friend of mine has MS and she needs to meter her energy levels like this. My neighbour came out of hospital after a serious illness and she has some of these symptoms. It is more common than you would think.

For me the mental fatigue is mostly related to context switching, being disturbed (slacks, meetings etc but also side quests to the original problem), and working on a fuzzy problem.

Working a few hours in such environment is very fatiguing.

On the other hand, when I work on a single thing, no disturbance, clear problem definition, having all necessary skills to do the thing, I can work 10h and it's not fatiguing.

It depends on the kind of work. If it's routine stuff, past seven hours or so, I can keep going and not feel tired, but I increasingly don't want to, and the feeling that I'd rather be doing something else becomes very distracting. If the work is technical and intellectually rewarding, I might feel inspired to continue, but I start making mistakes and past a certain point, it becomes counterproductive. If the work requires conceptual or creative insights, my brain stops delivering them for free and my backup methods for squeezing them out start failing too. If I'm speaking or writing, I lose the thread and my words lose their punch and personality. My occasional bouts of insomnia bring a different kind of all-encompassing fatigue. I become overemotional. At my most sleep-deprived, I struggle to operate a kettle. Things were different when I was younger. Writing up my PhD, I essentially slept every other night for months, yet stayed sharp, productive.

> Whereas my brain seems to be able to work at essentially the same intensity for as long as I want it to, up to 18 hours a day, and then I need a bit of sleep to recover.

Potential variable explaining this - what's your age? I could do this too in teens/twenties.

> How do other people here experience mental fatigue (or not)?

Just normal tiredness / distraction. Two days of actual full-on active pairing on something tricky and I just want to go to sleep after work.

Mental fatigue is when your brain is tired, but your body is not.

In other words, you don't want to sleep or lie down, you want to stop thinking.