Think about “reward” more broadly. When it comes to burnout, your body doesn’t care about money. Money is not a reward in and of itself, but rather it’s your feeling about money that matters.
When you experience burnout, it’s not due to muscle fatigue or lack of sleep. Burnout happens when you aren’t getting something that you need psychologically, over a long period of time. Then the psychological deficit begins to have physical symptoms.
I experienced burnout quite a bit in the first half of my life; hardly any in the second half. I’m 57.
The advice I got that helped the most is to listen to that little voice in my mind that was telling me “stop, I don’t like this!”
Also, I learned that when I am helping people and I feel respected in what I do my mind will happily endure whatever must be done.
Burnout is the result of a spiritual vitamin deficiency. I realize that is a metaphor for “chronic cortisol poisoning” or some biochemical thing, but the solution is to discover and stop the dynamic on the level of your mind.