Melatonin rhythmicity: effect of age and Alzheimer's disease.

The circadian rhythm of the pineal gland hormone, melatonin is generated within the hypothalamic suprachiasmatic nuclei (SCN), site of the circadian clock. The circadian clock and its output melatonin rhythm is synchronized to the 24h day by environmental light which is transmitted from the retina to the SCN primarily via the retinohypothalamic tract. Changes in both the amplitude and timing of the melatonin rhythm have been reported with aging in humans. Whether these age-related changes (reduced melatonin amplitude, earlier timing of melatonin rhythm) are a result of aging of the retina, the SCN clock, the pineal gland, their neural connections or a combination of some or all of these is not known. The fragmented sleep/wake patterns observed in the elderly and to a greater extent in patients with Alzheimer's disease have been shown to be partly related to an altered retina-SCN-pineal axis. Therapies designed to reinforce the circadian axis (for example, administration of melatonin or light) have been reported to alleviate the disturbed circadian rhythms and disrupted sleep. Future research needs to pinpoint the site(s) of age-related dysfunction so that therapies can be specifically tailored to correct the abnormality in addition to reinforcing any of the intact processes.