Circulating melatonin is metabolized primarily in the liver, and secondarily in the kidney. Melatonin has been used successfully in the treatment of insomnia and circadian rhythm sleep disorders. Several studies show that melatonin levels are lower in Alzheimer's disease (AD) patients compared to age-matched control subjects. If the expectation of melatonin activity in AD is to be neuroprotective, the treatment must be initiated at the earliest possible stage of the disease. There is substantial evidence that fragmented sleep, advanced sleep phase syndrome, insomnia, and impaired daytime alertness seen in advanced age are the result of brain dysfunction that is closely linked to disruptions in the regulation of circadian rhythms. The aging process is multifactorial, and no single factor seems to be of basic importance. An example of the effect melatonin has on aging is that in AD and mild cognitive impairment (MCI) patients.