Wingin'it; I'm pretty sure chocolate doesn't have melatonin in it. The problem is these brownies that have had melatonin added to them, that are sold and purported to be "relaxing" which contain melatonin, because it is not controlled by the FDA, and could cause medical problems for people who are not aware they contain melatonin. I saw a news story on Youtube about them a while ago, when a mother reported it because her children had bought them at a convenience store without realizing what was in them.

Caffeine is the culprit in chocolate that gets you hyper. And sugar as well, depending what kind of chocolate you're eating. Melatonin makes you sleepy, or so the producers of it say. Since it is not under FDA jurisdiction, no one has had to prove whether or not individual products have a clinically significant positive effect in order to sell it, because there is no government agency testing or regulating it. This is true of all herbal supplements and remedies, which is why they are sold over the counter and not by prescription.

Thankfully some places are trying to change laws about selling this kind of stuff, because it is a really lucrative industry that is not only bilking people blind with stuff that doesn't work, but also preventing people from seeking actual medical treatment because they believe the claims on the packaging.