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The release of melatonin, a hormone produced by the pineal gland, is controlled by the circadian clock in the SCN. Its levels rise during the night and decline at dawn in both nocturnal and diurnal species. Melatonin has been called the hormone of darkness because of this pattern. The SCN controls the timing of melatonin release; melatonin then feeds back on the SCN to regulate its activity. In mammals, for example, most of the brain receptors for melatonin are located in the SCN. Research has demonstrated that administering melatonin can produce shifts in circadian rhythms in a number of species including rats, sheep, lizards, birds, and humans. These effects are most clearly evident when melatonin is given in the absence of light input. Thus, for example, giving melatonin to blind people can help set their biological clocks. Melatonin is available as an over-the-counter nutritional supplement. Although claims are made that the supplement promotes sleep, the evidence for this is inconclusive. Potential side effects of long-term administration of melatonin remain unknown, and its unsupervised use by the general public is discouraged.