SALIVARY MELATONIN AS A CIRCADIAN PHASE MARKER - VALIDATION AND COMPARISON TO PLASMA MELATONIN

There are many situations in which it would be useful to know the phas e state of the biological clock. It is recognized that measurement of melatonin levels can provide this information, but traditionally blood has been used for the analysis, and there are many problems in extend ing the measurements into the home or field situations. The aim of thi s study was to develop and validate a salivary melatonin radioimmunoas say and to compare results obtained against a plasma assay for determi ning the onset of melatonin secretion. The assay developed was sensiti ve (4.3 pM) and required only 200 mu l of sample. A rhythm in melatoni n was detected in saliva, peaking at approximately 120 pM or 30% of th e plasma levels. Using an objective criterion for determining the onse t of secretion (mean +/- 2 standard deviations of three daytime sample s), the time of onset was shown to exhibit low intraindividual variabi lity (coefficient of variation = 1.5%-4.3%). The time of onset determi ned using saliva was significantly correlated with the plasma onset (r = .70, p < .05). The onsets determined were 22:30 h +/- 22 min for th e saliva and 21:50 h +/- 16 min for plasma for 17 subjects. Similarly, the acrophases of the saliva and plasma melatonin rhythms were signif icantly correlated. Neither posture alone nor changes in posture affec ted the calculation of the onset of melatonin secretion using the sali va approach. Very high saliva flow rates induced by citric acid result ed in lower melatonin concentrations compared to the gentle chewing on parafin film. These results firmly establish the use of salivary mela tonin measurements for phase typing of the melatonin rhythm in humans.