There is a major problem with melatonin that I have suffered and thought would be wise to share - in that if you take it for too long your body forgets how to produce melatonin. You can become dependent on it much easier than you think, like most things, it is a powerful hormone that is released in the body. Use it with respect and for no longer than 3-4 days max in my opinion.
It can also trigger very vivid nightmares in some. The reason for this is interlinked with how it can cause depression, as melatonin is synthesised to serotonin, and after long term use you deplete yourself of it as your brain starts to compensate for the extra available.
Many think that this is a very safe, simple and harmless "supplement" when in fact it is actually a very powerful hormone that the body produces naturally. Don't mess around with this stuff unless you absolutely have to!