Circadian Rhythms

Melatonin is a hormone produced by the pineal gland at night in both nocturnal and diurnal mammals. The circadian pacemaker imposes rhythmicity onto the pineal gland through a well-characterized neural pathway, thus driving the rhythm of melatonin secretion. In addition to this control by the circadian pacemaker, melatonin production can be suppressed by light. In seasonally breeding mammals, the nightly duration of melatonin secretion is used as an endocrine signal of day length, and seasonal changes in the duration of melatonin secretion times reproduction to the optimal time of year.

Because melatonin is produced at night, it was thought to be causally related to sleep in humans (although it should be noted that melatonin is produced at night in nocturnal animals, who sleep during the day). In fact, studies of exogenous melatonin administration have shown that melatonin can facilitate sleep onset at certain times of day, although not at all times of day.

Because there is increased sleep disruption with age and because of reports of exogenous melatonin's sleep promoting effects, there have been suggestions that an age-related reduction in melatonin level or a decrease in the duration of melatonin secretion might be associated with the age-related decrease in sleep quality. While some older people may secrete less melatonin, a study published in 1999 reported that nocturnal plasma melatonin concentrations in most very healthy older subjects was not significantly reduced when compared to those of healthy young men and women, nor was there a significant difference in the duration of the nightly melatonin secretion time between young and older subjects. Thus, neither decreased plasma melatonin levels nor a shorter duration of melatonin secretion can fully explain the age-related changes in sleep timing and consolidation that have been observed in even healthy older individuals.

There have been conflicting reports of whether exogenous melatonin administered to older individuals with insomnia improves sleep quality. In one study in which sleep (brain waves, eye movements, muscle activity) was recorded, of older insomniacs, exogenous melatonin administration did not affect total sleep time, sleep efficiency, or subjective sleep quality, and there was no correlation between endogenous melatonin level and sleep quality. However, another similar study reported that older insomniacs had low endogenous melatonin levels. That same study also reported that a low dose (0.3 mg) of melatonin improved sleep efficiency in those older insomniacs with low melatonin levels, although it did not affect the sleep in older control subjects. Thus, there is conflicting information about how endogenous melatonin levels in aging are related to sleep quality and whether melatonin replacement will improve sleep.