A study at Guy's and St Thomas's Hospital in London has found that those suffering from the syndrome - formerly known as ME - have up to twice the levels of melatonin as those who do not have the disorder.
Dr Theodore Soutzos told the conference that the discovery means that the syndrome "cannot be dismissed as psychological". He said that it was "not just about people whingeing", and those who succumbed to CFS tended to be people who drove themselves.
Dr Soutzos, who worked jointly with Dr Ram Seth, explained: "What happens then is that they get an increased workload - for example a working mother who has a couple of children to look after and then her own parents fall ill and she has to take care of them.
"They drive themselves until they overdo it, and then if they are susceptible to [the syndrome] start to produce high levels of melatonin."
The high levels disrupt the immune system, which leads to the patient getting ill.
The discovery means that CFS - sufferers from which include the Duchess of Kent and Esther Rantzen's daughter, Emily Wilcox - is not, as previously thought, a psychological disorder but is closely linked to immunology. The two authors of the paper called for CFS to be reclassified as an immune system disease.
The study looked at 44 CFS patients and 17 controls. Because melatonin levels can vary at times of day and at stages of the menstrual cycle, all the subjects were female at the same stage of their cycle.
Melatonin levels were double in those suffering the diseases and higher than in those who had been suffering from CFS but were improving.
Daily melatonin levels in the group suffering from active symptoms also remained raised for longer.
"Although there can be a psychological element to CFS, what this suggests is that people who suffer from it have an overactive immune system," said Dr Soustros. "It is rather like people who get gastric ulcers where psychology turns into biology."
Dr Soustros said that CFS was distinct from depression, which is characterised by low levels of melatonin, and that hopefully their discovery would mean that CFS patients would no longer be wrongly diagnosed with depression.