My son Christian has taken melatonin since he was 13. He's mentally handicapped and when he hit puberty, oh brother~! Every night was like a big ol' party in his room. He'd stay up singing and playing until the wee hours, then couldn't get up and go to school. He's like a 1yr old, so there wasn't too much we could do to fix this, other than try a sleep aid. The first doctor suggested a psychotropic drug but I wasn't comfortable with that. The second doctor told me to try melatonin, but if it didn't work we may have to resort to stronger medication.
It worked! The very first night he took it, he fell asleep 30 minutes later and slept almost the whole night! This was after weeks of disrupted sleep. I thought it was some kind of miracle! Now I dont' know if it works that well for everyone, but for Christian it has been a god-send. He is 18 now and he still takes the same dose (3mg) and as long as he takes his melatonin, he falls asleep and stays asleep. If we forget--Katy bar the door!
ETA: Melatonin doesn't have anything in it that will "knock you out." It's a hormone the body produces, the one that makes you feel sleepy after dark. When you take it, be sure you are ready for bed. Don't take it and then stay up doing stuff, especially computer stuff. That will negate the benefits of melatonin.