Not sure you want to change the light cycle on them, might disrupt their "normal" behavior. I would recommend something else besides the foam. If your using glass, which it sounds like you are, I put the standard decorative background around at least the back side of the tank, then I place this awesome material on the back of that. It's called reflexit, kinda like those car sunscreen thingys. Depending on your needs, in your case, shiny side torwards the heat source to keep heat in. I use it on one of my enclosures that's near a window and houses a BRB that has a big issue with temps higher than 80ish so I use that as relector to keep the heat off. I noted a 5 degree decrease in tempurature using the Reflexit, which keeps my proportional t-stat from working overtime and at the same time, keep the majority of the heat coming from the UTH, not the sun heating up the backside of the enclosure. If I were you, I'd cover three sides of the enclosures with the stuff, experiment with it. Hope that my rambling helps.