Researchers led by Professor Steve Atkin, an expert in endocrinology, came up with the idea of studying dark chocolate after a patient reported feeling less fatigued after she decided to swap her normal milk chocolate for dark chocolate with a high cocoa solid content.
Prof Atkin and his team then decided carried out a trial of 10 patients to check whether dark chocolate benefited them also.
As a part of the experiment, the patients received a daily dose - 45g - of dark chocolate or white chocolate dyed to look like dark chocolate for two months. This was followed by a month in which they were not given any chocolate, before being given the other type of chocolate for two months.
To their surprise, the researchers noted that patients taking dark chocolate reported significantly less fatigue when they started eating the sweet.
They also reported more fatigue when they stopped eating it.
Chronic fatigue syndrome (CFS), also known as ME, is a condition with a diverse range of symptoms but particularly characterised by profound muscle fatigue after physical exertion.
The experts say that this may be because dark chocolate may be having an effect on the brain chemical serotonin.
"Although it was a small study, two patients went back to work after being off for six months," the BBC quoted Prot Atkin, as saying.
"Dark chocolate is high in polyphenols, which have been associated with health benefits such as a reduction in blood pressure.
"Also high polyphenols appear to improve levels of serotonin in the brain, which has been linked with chronic fatigue syndrome and that may be a mechanism."
And, what's better, is that even though the results still need to undergo more tests, Prof Atkin says it is a win-win situation, for people would not harm themselves by eating small amounts of dark chocolate, nor put on weight.
"If you derive benefit, then it's a no-harm, no-risk situation," he said.