Chronopharmacological actions of the pineal gland

In all mammalian species studied to date, the pineal gland shows a pronounced circadian rhythm with high activity at night and very low activity in daytime, as reflected in the output of its hormone, melatonin. The pineal is one of the few organs which can synthesize melatonin and, under normal circumstances, is virtually the only source of circulating melatonin. Pineal activity is tightly controlled by the light/dark cycle, in such a manner that melatonin can convey information to the organism on the length of the dark phase of the photoperiod. In seasonal breeding species, the 24 hour pattern of secretion of melatonin shows seasonal changes which are crucial for determining the timing of seasonal changes in bodily function. In man, alterations in pineal function are of significance in jet-lag, shift work and in affective disorder.