Melatonin (N-acetyl-5-methoxytryptamine) is a hormone made by the pineal gland. It regulates the sleep/wake cycle also known as the circadian rhythm. Light suppresses melatonin secretion during the day to a level that is practically unmeasurable. Blood levels of melatonin begin to rise in the evening and peak around 3 am. Melatonin effects on the body are sedating, including a drop in body temperature, which promotes sleep.
Studies have shown that the active breakdown products of melatonin are decreased in people with insomnia, which suggests that melatonin suppression or deficiency may be responsible for some sleep difficulties. Melatonin is a hormone. It is one of a few hormones that are available without a prescription in the United States. Melatonin is available as an over-the-counter dietary supplement.