Permanent state of stress is a prerequisite to a burnout. If you are in touch with your feelings enough to confidently assess whether you are constantly stressed or not, you can probably catch it before you stop caring about saving the world. Top Google hits for "symptoms of a burnout" appear quite useful.

(Personal: going through the grad school: burned out twice, took a leave twice, still finished, because quitting was not a satisfactory option, but didn't pursue the same career once done.)