Melatonin

Melatonin is a hormone produced in the brain by the pineal gland from the amino acid tryptophan. The synthesis and release of melatonin are stimulated by darkness and suppressed by light, suggesting the involvement of melatonin in circadian rhythm and regulation of diverse body functions. Levels of melatonin in the blood are highest prior to bedtime.

Synthetic melatonin supplements have been used for a variety of medical conditions, most notably for disorders related to sleep.