If you're not relaxing after working, then you're doing something wrong!
There's PLENTY of time to relax and still accomplish everything that you're supposed to do for work.
Personally, my day is filled with 1 hour breaks throughout the day where I go outside for a walk, smoke a cigar and load up a video game, or go for lunch etc.
All work for 18 hours leads to being unproductive. However if you don't have much going on in a day, doing 15 hours of work can be a breeze when you break it up a little.
Granted, when a fire starts, there's no breaks, but really those kinds of instances should be rare too.