Daily and seasonal variations in the concentration of melatonin in the human pineal gland.

To elucidate whether pineal melatonin secretion is affected by changes in day length, we determined the concentration of melatonin in human pineal glands obtained at autopsy from 66 male subjects, aged 16-84 years over a period of 12 consecutive months. Based on the time of death, a day-night difference in pineal melatonin levels was evident only in the long photoperiod (April-September) with significantly higher melatonin concentrations occurring at night (2200-1000 h). Nighttime values in the long photoperiod were significantly higher than the nighttime values during the short photoperiod (October-March). During the short photoperiod, the data suggested a possible phase-delay in melatonin secretion. Day-night difference was evident in young subjects (30-60 years), but not in elderly subjects (61-84 years). Elderly subjects had lower total melatonin levels (day and night values) although statistically not significant. Therefore, melatonin levels did not decline with age and when the data were analyzed by age there was no significant day-night difference in melatonin levels. These data indicate that the concentration of melatonin in the human pineal is augmented only during the long photoperiod. The results suggest a partial effect of photoperiod on melatonin secretion in humans. This may result from living in an artificial light environment or due to other nonphotic signals involved in generating melatonin rhythm.