Under the main results section "Daily doses of melatonin between 0.5 and 5mg are similarly effective, except that people fall asleep faster and sleep better after 5mg than 0.5mg. Doses above 5mg appear to be no more effective." [1, from parent comment]

Upon further research with the link I provided. It seems that for older people dosage levels should be introduced small and then increased, this study talks about the effectiveness of 50mg in older patients [1]

[1] - https://www.ncbi.nlm.nih.gov/pubmed/24802882

I don't see anything in your comment about excess melatonin being evacuated.