The use of plasma melatonin levels and light in the assessment and treatment of chronobiologic sleep and mood disorders.

Using the highly accurate and sensitive gas chromatographic-negative chemical ionization mass spectrometric assay for plasma melatonin we have measured plasma melatonin in humans as a biological marker for 24-hour (circadian) and seasonal rhythms and the effects of light on these rhythms. We propose that there are at least three critical parameters for light to be chronobiologically active in humans: intensity, wavelength and timing. With regard to timing, we have found that bright light exposure in the morning advances circadian rhythms (shifts them to an earlier time) and bright light in the evening delays them (shifts them to a later time). We have suggested that chronobiologic sleep and mood disorders be "phase typed" into either the phase advance subtype or the phase delayed subtype and that these disorders can then be treated with either evening light (for phase advanced disorders) or morning light (for phase delayed disorders). Regarding the function of melatonin in humans, we have preliminary evidence that it may participate in the regulation of the circadian rhythm of intraocular pressure.