Mental health advocate @PJ_Palits posted a long thread on Twitter explaining why people with depression tend to be so tired. She attributes the fatigue to a number of reasons, and explains that just because other people don't understand depression doesn't mean that people don't truly suffer from it.
Not many people ask me if I’m OK, but when they do my answer is always the same “I’m fine, just tired” — and people seem to accept that reply.— ⓟⓐⓛⓘⓣⓢ™ (@PJ_Palits) January 20, 2018
For me, “I’m tired” is not a complaint or pessimistic. It’s merely a fact of life.