There's something in it though, I wonder if it's just as simple as how 'physical' it is - if you're sort of 'exercising', obviously anything manual labour is, but also walking around, reaching up to shelves, etc. then it's more continuously doable. Tired and hungry sure, but doable.

I couldn't keep my brain continually engaged in SE for even what would be a short 'shift' - but I reckon I could still do the >12h full days I did at the cinema as a teenager (plenty to do even while all screens are showing).