Chronobiological aspects of the mammalian pineal gland.

Pineal melatonin production is highly cyclic and depends on alternating periods of light and darkness. Peak melatonin synthesis occurs during the dark phase of the light: dark cycle. Interruption of the sympathetic nerve supply to the pineal gland, exposure of the animals to continuous light, or the administration of a beta-adenergic antagonist obliterates the daily rise in pineal melatonin levels. At least in hamsters, circulating gonadal steroids seem to have minimal influence on the circadian rhythm in melatonin production.