Investigations of melatonin secretion in man.

Overnight melatonin secretion shows substantial and significant intraindividual stability and large interindividual variation. Melatonin concentrations obtained between 1 and 5 a.m. significantly correlate with overall melatonin secretion (expressed as the area under the curve) while a single determination of serum melatonin does not adequately reflect the entire night secretion. Secretion correlates positively with "neuroendocrine responsiveness" (rating responses in other neuroendocrine challenge tests) and there is trend for negative correlation with age. In volunteers the administration of the usual therapeutic dose of lithium did not alter the overnight melatonin secretion. However, there was an elevation of isolated secretory values at the early morning timepoints. In comparison with volunteers, lithium administration tended to reduce the overnight melatonin secretion in bipolar patients, however, the difference did not reach statistical significance.