Actually, you can choose the sleep position that is most comfortable for you. It's just as simple as that. There is no recommended sleep position, or a position that is unadvisable.
In fact, the common belief that bed rest is necessary after an ET is a myth that can be counterproductive, as it increases the levels of stress in the woman. It is erroneous to think that gravity can make the embryos fall out or reduce their possibilities of implanting.