Melatonin is a hormone produced in the brain by the pineal gland. It is a derivative of the amino acid tryptophan. It plays an important role in the regulation of sleep cycles (i.e., circadian rhythm) with levels rising in the evenings to help prepare our body for sleep. They then drop during the early morning hours, helping prepare us for wakefulness.
There are many other actions of melatonin within the body which include: