This is a common problem, not only in CFS, but in other chronic diseases as well. Although we do not have a detailed explanation of this, the situation certainly improves with successful treatment of chronic bacterial and viral infections. I suspect that it is caused by wide swings in hormone levels, which are in turn caused by infectious changes in endrocrine gland cells, like nerve cells, a target of infections like Mycoplasma and Chlamydia species. ProfGN