Melatonin as a marker and phase-resetter of circadian rhythms in humans

With the advent of RIAs capable of measuring the melatonin onset (60), the DLMO has been increasingly utilized as a marker for the phase of the ECP. Measurement of melatonin not only helps assess circadian phase but also has been important in developing the use of bright light to assess light sensitivity, as well as to shift the ECP in the treatment of circadian phase disorders. Physiological doses of melatonin have been used to describe a PRC that is about 12 hours out of phase with the PRC to light. Melatonin treatment of winter depression may eventually provide conclusive proof of the PSH for this disorder. One function of endogenous melatonin may be to augment entrainment of the ECP by the light/dark cycle. Another function may be to serve as a nighttime darkness discriminator. The fact that melatonin is generally made only at night and can be suppressed by light have been the guiding principles behind these developments. The early studies were also made possible because of the highly accurate and sensitive GCMS assay for melatonin (61).