I discuss melatonin's effects on sleep & its safety; I segue into the general benefits of sleep and the severely disrupted sleep of the modern Western world, the cost of melatonin use and the benefit (eg. enforcing regular bedtimes), followed by a basic cost-benefit analysis of melatonin concluding that the net profit is large enough to be worth giving it a try barring unusual conditions or very pessimistic safety estimates.
Full essay: http://www.gwern.net/Melatonin
It could be that you weren't taking enough. Another major factor is your bedtime routine. Exercise, bright lighting, and other stimulating activities (video games, emotionally-engaging movies) can discourage sleep.