Melatonin has been a wonder drug for me. Been taking it for almost a decade now, sleep like a baby. Can have a cup of coffee at 8pm (yes I do that), pop one at 9:30 and be sound asleep at 10. Basically gets me really tired after 10 minutes.
But also has 0 effect on a lot of people I know. Think it's great for traveling too, changes your body clock and gets rid of jetlag.