Intrinsic rhythms of the pineal gland and associated hormone cycles in body fluids

Clearly, virtually any parameter of pineal biochemistry that is investigated exhibits a 24 h fluctuation. Many of these rhythms are synchronized by the prevailing light:dark cycle. Relatively few, however, have been investigated under anything other than usual laboratory long day photoperiods. Whereas there is little doubt that the rhythms relating to pineal serotonin metabolism are interrelated, there are a number of other cycles in the gland that seem to have little to do with rhythmic melatonin production. Also, whereas we know a great deal about the mechanisms controlling the cyclic production of melatonin, factors governing the other cycles have been only meagerly investigated. At the beginning of the resume the author made the comment that the pineal gland is a chronobiological haven; it is the hope of the author that this review convinced the reader of this point even though not all the rhythms that have been observed in the pineal gland were reviewed in this brief summary.