| Melatonin is a hormone produced by the pineal gland, discovered in 1958. It's chemical name is N-acetyl-5-methoxytryptamine. Melatonin has been popular for the past ten or twenty years as a sleep aid, and pretty much everybody -- that is, both scientists and people who have used melatonin successfully -- agree that a reasonably sized dose will make the transition into sleep easier. This effect is unlike that of a depressant, as it doesn't make you feel disoriented and loopy, it just makes you feel like sleep would be a very good idea.
Use as a sleep aid is desirable for people suffering from light or moderate insomnia. Melatonin may also used by those who want to reset their biological clock, to ease jet lag or the transition to new working hours, etc. Usual dosage for insomnia is around 3 mg; for jet lag, or any other time when bio clock reset is desired, 10 or 15 mg may be taken without danger.
Melatonin is produced constantly, but the pineal ramps up production tenfold when the sun sets (ie, when the amount of visible light goes below a certain threshold), at around 6 pm. Peak melatonin serum levels are reached between 2 and 3 am, and production slackens back to daytime levels by 8 am. The pineal converts melatonin from serotonin (5-HT), which is produced from tryptophan in other parts of the brain. Melatonin levels in women also change with menstrual cycles -- it is at highest concentration during the premenstrual period, and at lowest during the midmenstrual. Also of interest is that rhythmic melatonin production is completely absent in babies until about 3 months, partially explaining their non-existent sleep cycle.
Due to it's near inability to be overdosed upon, it's relatively high concentrations in untreated animals and humans, and the subtlety of its effects, melatonin is classified as a food supplement rather than a drug. While this means it's cheap and easily available, it also means that no FDA testing has been done with regard to its side effects, and that there is no regulation of what manufacturers put in a bottle of pills marked "melatonin." As with everything, buyer beware.
Pop medicine zealots like to proclaim melatonin as everything from a cure for cancer to a method of life extension, but there are few studies on any effect other than that of a sleep aid. Melatonin is, however, known to be an effective antioxidant, and has a particular affinity for the toxic peroxyl (H2O2) and hydroxyl (.OH) radicals. Seasonal affective disorder (SAD) may also be treatable with melatonin, as it comes from the "confusion" of the biological clock caused by the darkness of winter.
Many, many people report increased dream activity while using melatonin. In the (admittedly informal) study I read, 63% of the people taking the survey reported increased dream activity, and 88% reported increased vividness of dreams. Interestingly, 50% also reported that melatonin influenced the content of their dreams -- the dreams were perceptibly different from their usual ones. | 
