Man Sleeping with Visible Brain : The sleep-wake cycle is an important circadian rhythm. When it is dark, the suprachiasmatic nucleus directs the pineal gland (located in another part of the brain) to secrete melatonin. This hormone peaks at night and ebbs during the day, and plays a key role in the sleep-wake cycle. Light disrupts melatonin production, and individuals who are exposed to too much light in the evenings, or who work the night shift, may experience problems sleeping as well as disruption in the menstrual cycle and other systems.

Melatonin, chemically N-acetyl-5-methoxy tryptamine, is a substance found in animals, plants, fungi, and bacteria. In animals, it is a hormone that anticipates the daily onset of darkness; however in other organisms, it may have different functions. Likewise, the synthesis of melatonin in animals differs from that in other organisms.

In animals, melatonin is involved in the entrainment (synchronization) of the circadian rhythms of physiological functions including sleep timing, blood pressure regulation, seasonal reproduction, and many others. Many of melatonin's biological effects in animals are produced through activation of melatonin receptors, while others are due to its role as a pervasive and powerful antioxidant, with a particular role in the protection of nuclear and mitochondrial DNA.

It is used as a medication for insomnia, however, scientific evidence is insufficient to demonstrate a benefit in this area. Melatonin is sold over-the-counter in the United States and Canada. In other countries, it may require a prescription or it may be unavailable.

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