I think 40 hours / week is fine and sustainable, as long as you're not expected to be 100% throughout. That amount of caffeine is insane and (I think you know this yourself) a patch for something else that you're missing, like normal food and a good amount of sleep per day.
Honestly I don't think 40 hour weeks were in any way fine or sustainable for me.
Only when I reduced to 35 hours i felt I have productive time next to work. And today I maybe work 4-8 weeks a year at 60 hours and the rest closer to 10 hours a week.
I am not sure how other people cope with that, but 40 hour weeks are crazy time consuming in my book.
