The Pineal Hormone-Melatonin

Abstract

The pineal gland or epiphysis cerebri (Fig. 1) is a neuroendocrine organ that exerts important regulatory influences in vertebrate animals by secreting its hormone, melatonin (Fig. 2), in variable amounts, depending on the animal’ s age, the time of day, and in some species, the time of year. The daily rhythm in plasma melatonin concentrations (Fig. 3), which is low during the day and high at night, is probably a significant factor in sleep initiation and maintenance. The decline in nocturnal melatonin levels during adolescence may be a factor in the onset of puberty; the further decline observed with advancing age (Fig. 4) may be a causal factor in age-related insomnia. In some mammalian species, seasonal changes in the number of daylight hours each day influence serum melatonin levels, and thereby seem to trigger the annual recrudescence and, later, atrophy of the gonads. Daily and annual rhythms in serum melatonin levels may also affect the timing of other biological rhythms, for example the day–night rhythm in body temperature and locomotor activity.