I used to take 3mg melatonin -- this used to work for a day or two, but then it stopped working. I switched to 300mcg or 450mcg and it just keeps working, no t

I used to take 3mg melatonin -- this used to work for a day or two, but then it stopped working. I switched to 300mcg or 450mcg and it just keeps working, no tolerance whatsoever (actually, after 4-5 days of getting into a sleeping routine with melatonin, I find I no longer need it and I can just fall asleep on my own). 300mcg means I don't notice the melatonin working, I just fall asleep. 450mcg of melatonin -- I actually feel it kicking in like I'm a child wanting to stay and watch the movie, but I just can't stay awake.