To sleep mouth down is not recommendable, since when doing it usually it modifies the curvature of the lumbar column, and to be able to breathe the neck turned during several hours

To sleep of flank is an intermediate option that allows a good rest if it is made suitably. In any case he is recommendable to change frequently of position.

En todo caso es recomendable cambiar de postura frecuentemente.

The best position is to locate mouth arrives, supporting all the column on the mattress. With quite flex legs.