"I'm curious what those with some knowlege think that a possible
negative reaction could be from using 3 or 4 mgs of melatonin every
night for ten to twenty years.

I'm not saying there should be some, just curious if anyone has read
anything or may postulate something."

I have done a rather extensive search of the melatonin literature for
personal reasons.  There is nothing that jumps out except the usual
caution that long term studies in humans is lacking.  I have also done
similar for aspirin and it clearly has potential effects for long term
high dose use, even in the short term.  I offer this as a reference when
comparing it to the melatonin studies.

Probably a lot less worse than using the standard dose of Ambien every
night for even 3 years.

Just *the usual*, which makes sense to me and is why they say to be
careful with it.

That is, that you'll make less and less of it (a bad thing!) as you age
without intentionally messing up what appears to be a rather delicate
mechanism.

It's like growth hormone or testosterone in that respect -- are you sure
you want to try "outsmarting" your long-evolved endocrine system with
something that was only discovered recently and for which we have no
long-term experimental data on the consequences of oral supplementation?

All the advice I've come across on melatonin concurs: that taking it
rarely, to help nudge your sleep cycle into coping with jet lag or (more
controversially) seasonal affective disorder, isn't likely to
precipitate a permanent reduction in its usual elaboration in the brain
in response to darkness, _unless_ there's some tendency in that
direction already (a "big if").

But it sounds to me as if you might be considering taking _high_ doses
of melatonin over a period of one or two _decades_, entirely without a
doctor's advice, in spite of plenty of warnings everywhere that that's
crazy!

Furthermore, you make no hint of recognizing any _indication_ for doing
this.  I assume it has something to do with sleep or "nootropy", but
don't you think you should at least establish what your baseline
melatonin levels are _without supplementation_ before risking your
health by intentionally altering them?  You wouldn't start injecting
yourself with insulin on a hunch, would you?  Well, just because
melatonin doesn't have to be injected and can be bought (at present)
over the counter doesn't make it a good candidate for playing around with.

I realize I'm jumping to conclusions here, but it's only because I'm
(having fun and) addressing unknown people besides yourself.  I hope
you're not personally engaging in a dangerous practice, paramindsoftware!