Most managers already know that you can't work 100%. They expect 20% of your working week to be spent on things which support work but isn't directly the work

Most managers already know that you can't work 100%. They expect 20% of your working week to be spent on things which support work but isn't directly the work task itself (meetings, communication, HR, training, etc). And as this is spread out across the week it's not as obvious and visible to the worker that this is the case. One can still work unhealthily hard for those 4 days a week of real effort. I share your opinion that you should only work that hard 80% of the time (he says, as he starts another 12 hour day), but if you're not accounting for the stuff around the work that still adds up to working at 100% capacity and without the breaks to keep yourself sane, healthy and developing.