There is no diagnostic test for chronic fatigue syndrome and is really is a disease of exclusion.
In many cases patients are diagnosed with chronic fatigue syndrome but turn out to have serious immune system problems, cancer, dysautonomia and other.
Also, the drowsy, dragged down feeling, after having been very high.
See Chronic Bronchitis.
"I'm startin' to come down. If I dont smoke some more my Chronic Fatigue will set in."