Back in the late 1980's, I was heavily involved in the flight test program for the 767 re-engine program. As I result, there was a month where I flew nearly every day (flight testing is 7-day/week program, so no assurance of getting even one day off - since then they implemented a rule that required at least on day off every 14 days but that wasn't the case then).
Thing is, on most of those flights I didn't do anything besides sit in the back - either reading or looking at the view out the window - and munch my flight lunch. My testing was 'contingency' - if they couldn't find the calm air they needed for the various aero such testing planned, they'd run my tests instead. So long as they could find 'air', I literally had nothing to do.
By the end of that month, I was a zombie - totally exhausted (and I was in my early 30's at the time so it wasn't age). It simply amazed me that I could get that exhausted just sitting. I assumed that stress was a big part of it - flight testing is inherently stressful - although nothing we did was considered 'high risk' - flight testing was incredibly expensive (I heard numbers of $50,000 hr. - probably double that today), so everything was always 'rush-rush'. If someone asked you a question about your test, you didn't get 15 minutes to think about it, they wanted an answer right NOW! Plus the whole environment was stressful - you could almost feel it in the air. Worse, I didn't sleep well - between the residual stress and lack of physical activity, my brain was fried but my body wasn't that tired...