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 (/ˌmɛləˈtnɪn/), chemically N-acetyl-5-methoxy tryptamine, is a hormone found in animals, plants, fungi and bacteria in anticipation of the daily onset of darkness. It is synthesized in animal cells directly from the amino acid tryptophan, but in other organisms through the Shikimic acid pathway.

In animals, melatonin is involved in the entrainment of the circadian rhythms of physiological functions including sleep timing, blood pressure regulation, seasonal reproduction and many others. Many biological effects of melatonin 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.

The hormone can be used as a sleep aid and in the treatment of sleep disorders. It can be taken orally as capsules, tablets, or liquid. It is also available in a form to be used sublingually, and as transdermal patches. There have been few long-term clinical trials in the use of melatonin in humans.

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