The study included 185 Georgia adults, 75 of whom had chronic fatigue syndrome. Those patients had fatigue lasting at least six months with no known cause and accompanied by at least four other symptoms, such as muscle pain or memory problems.
Participants provided saliva samples taken as soon as they woke up, and again 30 minutes and an hour later. The CDC's William Reeves, MD, and colleagues measured cortisol levels in the saliva samples.
Chronic fatigue syndrome was associated with lower morning cortisol levels among women, but not among men. Morning cortisol levels were similar for men with and without chronic fatigue syndrome.
The study doesn't prove that low morning levels of cortisol cause women's chronic fatigue syndrome. The researchers don't know which came first -- low morning cortisol levels or chronic fatigue syndrome -- but their findings may be a clue for researchers.
The study appears in The Journal of Clinical Endocrinology & Metabolism.