I taught in secondary schools for 35 years, the last one for 33 2/3rds. I realised it was time to retire when I was getting up at 5.15 and in work for 6.30. A typical day meant leaving at 4.30 after having a couple of 15 minute breaks, no lunch time and taking an hours work home. Add to that 1/2 a days work over the weekend and I'd ha enough. I was constantly exhausted.
Now retired, at 56yrs old I get up about 8.00 but still do10 hrs a day 7 days a week on a retirement refurb of a bungalow. The thing is although I'm tired I feel good.
Colin