I have been using the supplement Rhodiola for fatigue and mental fatigue. What I can say is that the research is great for fatigue. I can also say that at my dose of 105mg I do see the benefits to it, but I believe it would be better twice a day. It seems to help after I take it for a certain duration and then I slump again. Although, with both fibromyalgia and migraines I can see why I would need an additional boost. Nevertheless, it is noticeable which is rare for supplements.
Research suggests that it significantly reduces fatigue. And that in people who are fatigued helps with cognition issues, related to the fatigue... in other words, clears up the mental fatigue cloud. For a daily preventative of fatigue, it can be beneficial in as low a dose as 50 mg. "Rhodiola can improve cognitive functioning in persons who experience a reduction in fatigue, but there is insufficient evidence to support an outright increase in cognition (which should occur in non-fatigued persons)" Examine.com