You're not just standing watch, though. There's training (SO MUCH TRAINING), drills (SO MANY DRILLS), meals, and at least a little time for movies/relaxation. Trying to fit all this in is more complicated with 8 hour watches. (With 6 hour watches, you do all training/drills in the 0600-1200/1200-1800 time periods, and it works out easily.)