How come a lot of places say that in a study, humans had 6 grams of melatonin every day for a month and were fine? where is this study?

Melatonin dose: I hope you mean 6 mg. The typical melatonin dose is 3 mg (milligrams, or 1/1000 of a gram). 6 mg would not be a big deal. I can't imagine what 6 grams would do. That would be 2000 pills. If "places" say something ("studies have shown...."), and they do not cite a reference that you can check out, be suspicious.

Milligrams not grams: You mean milligrams, not grams. Patients should use the smallest dose of melatonin necessary for symptomatic relief. Many studies have investigated doses of melatonin at a 6 mg/day or greater dose (one example: http://www.ncbi.nlm.nih.gov/pubmed/107392990. Discuss with your sleep specialist before making any medication dose changes. http://www.nlm.nih.gov/medlineplus/druginfo/natural/940.html