Melatonin (N-acetyl-5-methoxytryptamine) is a hormone found in a variety of species, and in mammals it is a part of the sleep-wake cycle.
Melatonin is secreted by the pineal gland in response to darkness, and production is inhibited by light (specifically light in the blue spectrum). It promotes drowsiness and decreases libido in females by inhibiting the secretion of follicle stimulating hormone (FSH) and luteinising hormone (LH). Because of its proposed ability to promote sleep it has been used in sleeping tablets.
 As a sleep aid
A dose of as little as 0.3mg has been shown to be effective at treating certain types of insomnia. There are many melatonin supplements available sold as sleep aids, however most contain 10x or more the required dose and can actually prove to be counter-productive.
 Other medical correlation
Wonder brain chemical or not, the medical establishment has just begun its research into the effects of melatonin on the body's function, and correlative (vs. causative) links have been established with a host of modern medical problems including: long term clinical depression, migraines, diabetes cycles, attention deficit disorder in children, Tinititus, weight-gain and loss, anxiety disorders, and even some cancers. It's unclear if these are correlations with the associated lack of sleep common to a lack of melatonin production, or with the chemical itself but research on melatonin shows that modern interrupted sleep cycles have contributed greatly to our own medical bills.
 Night Milk
A company in Germany has taken to milking cows in the early hours of the morning, producing milk that is purportedly 25x times higher in melatonin than regular milk. The milk is then marketed as "Night Milk" or "Nocturnal Milk" as a sleep aid.
- ↑ Melatonin, sleep, and circadian rhythms: rationale for development of specific melatonin agonists
- ↑ Low, but not high, doses of melatonin entrained a free-running blind person with a long circadian period
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/21861099
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/21860691
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/21859418
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/21859051
- ↑ German night milk may aid insomniacs