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
The fact that the study was on children certainly doesn't help the validity when applied to adults, but I think you're being overly risk-averse. Melatonin's mechanism of action is pretty well understood, and it occurs in the body already. The long-term effects would have to be very bad to outweigh the advantages of a regular sleep schedule and an extra hour of wakefulness every day. That's assuming melatonin works, of course.
Endorphins are chemicals that occur naturally in the body, with a mechanism that is pretty well understood. Yet taking opioids regularly is not good for you.
You cannot assume health-benefits simply because it already occurs in the body.
There may well be benefits, but they must be proved independently of simply understanding the natural mechanism.