You ask a good question. I can only presume artificial light exposure does not impair their migration because I do know that they have recovered tagged monarchs at their overwintering grounds. So monarchs raised in a controlled environment, which presumably has artificial light, do migrate. In the video the science teacher posted I learned that they will form their chrysalis away from the light. In effect they would all be facing the same way. I mess with this desire as I never paid attention to which way I put the lid back on or which way I face the cage when I'm done cleaning it. I'm now going to make sure I don't mess with the direction of the lid or cage so all chrysalis can form in the manner they desire.
I am also going to cover the cages at night with a towel to diminish the light pollution they receive. I'll only place over half of the lid so that they can get oxygen.
In the end, we are probably impairing them somewhat if we don't know everything about them.
I do know that artificial light wreaks havoc on wildlife so your question would prompt a logical response that the cages should be covered from light. I just haven't thought about it until you asked it.