Talking to my doctor & physiotherapist they always said it's not necessarily the length of time you work it's how you work and how that time balanced between work focused time and self-focused time plus exercise. The worst thing you could do is stay immobile totally work focused. Guess what pilots do.
Imagine sitting at your desk for 10 hours looking out of the window on floor 30, looking at your computer screen and doing some work, answering the telephone a few times, chatting to your neighbouring desk, back to the screen monitoring that your previous work was taking effect - adjusting as necessary, accepting a cuppa delivered by your secretary, taking a pee break every 2 hours and a 5 minute stroll round the office before back to the seat and screen again, making more work related entries to react to previous work, a few stolen minutes with the newspaper or internet.
Now do that for 5 days starting at 06.00 or 5 days until 23.00 including weekends. So little exercise, very static. No balance. Your overall fitness would suffer; so why shouldn't ours? Very unhealthy according to my doctor & physiotherapist, yet supposedly the medical profession say it's OK for pilots, or at least that's what EASA muppets tell us. If an office worker went to the company doctor with fatigue and a sore lower back I'm sure they would be advised to change their working life style and mover about more: hence the 'break' every 2.30hr and lunch break for office workers. Health.