The Responses of Melatonin Rhythms to Environmental Lighting
Two very powerful generalizations characterize present conceptions of normal pineal function (1–3): In almost all species examined thus far in laboratory situations, the pineal secretes more melatonin at nighttime than during the day, and melatonin secretion is suppressed when the animals are placed in a lighted environment. Since nighttime for humans coincides with the daily dark period, investigators have often tacitly assumed that the second generalization explains the first, — that is, that the circadian increases in melatonin secretion occurs nocturnally because the absence of light terminates the retina-mediated suppression of pineal sympathetic activity, thereby increasing the organ’s stimulation by norepinephrine.
KeywordsMelatonin Secretion Melatonin Synthesis Laboratory Situation Melatonin Rhythm Daily Light Period
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