Rats will sleep in different positions depending on how they feel.
For example if it's hot in the place they are being kept they will flatten themselves as much as possible and lay splayed out and not in their beds. Although some rats will sleep like this for no reason.
If there are more than a few rats in a cage together, they will sometimes lay in what we call rattie piles. Where you would think the one on the bottom can't breathe but they are just fine and comfortable.
Lucky in the above picture has started laying on his back for whatever reason. He's been doing it for sometime now and I guess he just found a new position he really likes.