This is what burnout is. Burnout is not caused by working, but by stress. The root cause of this stress is personal and interpersonal factors. People can burn out on very small workloads if the ratio of interpersonal toxicity to hours of work is high enough.
Sometimes it looks like people are burning out because of extreme workloads. In these situations, I've always asked "why is the workload so extreme to begin with?" This gets you to the real source of stress.
It's amazing how many issues in life are caused by the act of thinking of these possible issues, than the issues themselves. My friend had horrible issues sleeping, he tried many things before going to psychiatrist. The conclusion was quick and accurate - your issues sleeping are caused by your fear of not falling asleep, basically anxiety. Anti-anxiety meds fixed it immediately.
Sometimes it looks like people are burning out because of extreme workloads. In these situations, I've always asked "why is the workload so extreme to begin with?" This gets you to the real source of stress.