A long and serious illness is an example of something that would be described as enervating.
At the same time his rule, if not harsh, was enervating and demoralizing.
I hope to be a friend that is encouraging, not enervating.
The first week of classes was enervating to the new students.
Warm, sunny days can be very pleasant to relax, but also very enervating.
The climate, especially from November to April, is somewhat enervating to the Englishman, but not unhealthy.