Melatonin will definitely help you fall asleep. When I am not sleeping at night I usually take melatonin and it knocks me right out.

On a negative side, too much of it makes me groggy. Too little of it induces a waking-sleep for me and causes me to hallucinate (outside of my normal night-time hallucinations). A half a 5mg pill usually does the trick to make me fall asleep and stay asleep.

My SO, who has PTSD, takes it to help him sleep at night (he doesn't like medications) and it works like a charm for him. No negative side effects.