You don’t just sit there for 12 plus hours. You sit for 6 hours, and then you’re horizontal for the duration in a very quiet and fairly comfortable bed. When I get to the layover I usually take a nap, exercise, and go have a healthy meal. After that it’s a good night sleep, a nice quiet breakfast, and then repeat. I feel much better doing this than when I was up and down multiple times a day, airport appreciation breaks, multiple bag drags to short layovers, etc. The difference in health stats is how people take care of themselves, not what type of flying they do.