That is a great question!
I have wondered the same thing myself. Of course, 'inside or out', is not as important as length of daylight hours. I know that light has an amazing effect on many types of plants, and some varieties of birds. In the classroom, everyone goes home at 4 or 5 and the lights go out at dusk. so caterpillars raised there are getting natural light. At home, the lights might be on until 10 or 11. I began putting my "home" caterpillars out-of-doors for the last brood or if indoors, I try to have lights out at dusk. I have no idea if it makes a difference with Monarch caterpillars. Probably the Monarch Watch scientists do!