There are some threads here about this, and the consensus is that it takes about 3 times screwing with the dark cycle before the plant stresses to the point of hermie, which is the main problem when you mess with the dark cycle (besides your plants not flowering at all)

It's recommended that your dark cycle is so dark, that you cannot see your own hand in front of you. Yes, in nature the moon puts out a little light... so the only way to know if that reflected light has any effect is to wait for flowers. If your plants flower, then it should be ok.