If the room is really dark during the day it is probably confusing their internal clocks. That's what I'd think atleast! Try opening a curtain and a blind and letting some natural light in during the day. Normally, once it starts getting really light, they will go curl up to sleep.
But of course a vet check is always in order when they start acting weird! I'd pretty much bet that their sleeping habits are due to the room being so dark BUT since they haven't been to the vet yet since you got them, they need a little check up anyway.
Welcome to the boards and the wonderful world of suggies!!! Got any pics?