Strenuous exercise increases plasma melatonin, cortisol, and ?-endorphin concentrations. Furthermore, a relationship between endogenous opioids and melatonin has been proposed. We measured plasma melatonin, cortisol, and ?-endorphin in 46 subjects before and after a 28.5-mile high altitude race. Thirteen of the subjects received the orally active opioid antagonist naltrexone immediately before the race. The mean plasma melatonin, cortisol, and ?-endorphin levels were higher after the race than before it; the melatonin results were confirmed by gas chromatography-mass spectrometry assay of 12 subjects. Naltrexone had no effect on the increase in any of the three hormones. (via.)
This study was briefly mentioned in “DMT: The Spirit Molecule.” I happened to run across it and thought I’d tuck it away on the blog in case it ever comes up again for some reason.