EDN1 and myalgic encephalomeyelitis/chronic fatigue syndrome: Endothelin-1 (ET-1), a strong endothelial vasoconstrictor, has been related to the regulation of the circadian rhythms [21], and high circulating ET-1 levels have recently been reported in ME/CFS [19]; thus we hypothesized that there would be an alteration of the vasoconstrictor/vasodilation process, which might be reflected in temperature variations, and would indicate dysautonomia in ME/CFS.