I speculate that burnout is more psychological than physical. Ultimately, I think it stems from the shame arising out of the realization that giving 110% instead of 70% at your job accomplishes little but making things harder for yourself. A software engineer doesn't get more money giving 110%, and if they work to finish something quicker they're just rewarded with more work. Often engineers (being human) will want t…
Some burnout is basically your mind slowly coming to associate some (class of) activity with negative feelings eventually building up to the point where it basically becomes an emotional net negative to the degree you can't push yourself to work on it anymore and you (have to) disengage.
Then there's the kind of burnout where you've been marinating your nervous system with extreme amounts of stress hormones for extended periods of time (months or more likely years on end) and when your performance starts to degrade due to this, as it inevitably will, you keep pushing yourself to maintain your expected standard of productivity up to the point where your body is basically trashed and your productivity suddenly drops precipitously to virtually non-existent levels which generally takes years to recover from.
The former is mostly psychological; the latter starts psychologically but most definitely crosses into the territory of the physical.
As an aside; the book The Upside of Stress [1] is a really good book on dividing 'good stress' which basically trains up your body and 'bad stress' which bathes you in damaging hormones. Many burnouts happen when someone with a high workload unwittingly switches from good to bad stress.
[1] https://www.goodreads.com/book/show/23281639-the-upside-of-s...