This topic holds some separate things, and it helps to keep them apart. Load is how much you are carrying. Limits are where you draw the line. Stress is what your body does with the result of you not being honest about the first two.
You can carry an enormous amount without strain if you have real control over how and when and mostly: what. People get tired from working to much but nobody got a burn-out from doing what they love. Interestingly enough most people that do what they love can spend the majority of the day creating what they need to create.
The clearest signal is what you drop first when things tighten. Almost nobody drops obligations. They drop the walk, the book, the friend they meant to call, an hour of sleep or the rest they need. Basically what one really needs comes often at the last place.
Then there are the tells. Shoulders that migrated towards your ears sometime in March and stayed there. A deadline finally met, and instead of relief a strange flatness is felt, because the next task is waiting. A holiday that takes four days to start working, by which point it's almost already over.




