“But... Doesn’t this study show strong evidence that it *is* all in the mind?”
Insufficient blood flow to the brain caused by failure of the brain, heard and autonomic nervous system to regulate blood pressure when the body and/or brain are subjected to a stressor. For those with severe CFS, going from a supine to sitting or standing posture is sufficient to cause loss of blood to the brain in a relatively short time. This has nothing to do with lack of sleep as a primary cause.
I notice that throughout most of my worknight: when I get up from a crouch, more often than not I feel lightheaded.