I haven't read the study, just the abstract, but a review of literature to attempt to answer the question 'is CFS a sleep disorder?' seems like a useful thing to do. The conclusion seems helpful. Full text is free access.
Umm, clearly it is possible to define CFS in this way, but if you do, you are wrong. (Incidentally, the English in this paper is poor. It's distracting; it should be better in an English language publication. It's published in E-Cronicon - the homepage says: which isn't reassuring.) Really? Some interesting studies are discussed; but it's very patchy. I know there are more CFS sleep studies out there than are reviewed here. It doesn't get into questions of sleep architecture (deep sleep/REM sleep). Pasting this here as yet another example of cortisol levels not being found to be abnormal. This article reads like a literature review done prior to a study using fMRI. The lead author's details are as follows:
