I am not an AME or any other kind of doctor but...
According to this
UK CAA text, Melatonin "
is not acceptable for EASA medical certification."
However,
CAP 789 (Requirements and Guidance Material for Operators) seems to suggest the opposite...
"
In the light of what is now known about melatonin, its use by flight and cabin crew is not recommended. Because melatonin will cause sleepiness and impair performance immediately after ingestion, and because the after-effects could be detrimental to operational efficiency, operators should forbid its use less than 12 hours before the start of a flying duty period and during the duty period."
I suggest you talk it over with your AME. He or she will be able to give you the best advice.