Melatonin naturally occurs in your system, hence it's not a downer or depressant.

It helps to regulate your circadian rhythms, which in turn can help regulate your sleep patterns. It is not so much an aid to sleep onset or sleep maintenance as prescription sleeping medications are. Since it naturally occurs in your system, there is less chance of addiction and overdose, though you should follow the directions on the bottle. Also, you can increase your own levels of melatonin by getting more sunlight, since that is the way it produced in your system. Low levels of daily sunlight will usually mean you'll have low levels of melatonin. Conversely, the more daily sunlight you're exposed to the higher levels of melatonin you will have in your system.