regarding a day room: I think the Ibis Hotel has day rooms as well, and it's a lot cheaper than the Sheraton.... I think it was about 40€/3h or something like that. You can book those by using the courtosy phone landside in the terminals (remember, the people on the other end of the line are not native English speakers, so speak slowly and use a simple English!)
The transfer to the IBIS hotel is very easy and can just be done via the CDGVAL people mover.