Melatonin rhythm in human plasma and saliva.

Human plasma and saliva were collected at frequent intervals throughout the night and after a nocturnal challenge by exposure to 3,000 lx of light for 1 h in the middle of the night. Melatonin, as measured by radioimmunoassay, was found to correlate highly in plasma and saliva, described by a linear regression equation: y = 55x-2.6 (r = 0.90). The nocturnal melatonin rhythm in saliva was parallel to that observed in plasma. A good correlation was also observed between plasma and salivary melatonin on exposure to light. Melatonin in both fluids showed a significant fall during light exposure. Levels returned to normal nocturnal values within 2 h after returning to darkness. These results indicate that salivary melatonin, although lower than plasma melatonin, may be used as an index of pineal gland release of melatonin. It is suggested that saliva may be useful as a non-invasive technique for obtaining data on melatonin profiles, especially in pilot-test and screening situations.