A sleeper wall is a short wall used to support floor joists, beam and block or hollowcore slabs at ground floor. It is constructed in this fashion when a suspended slab is required due to bearing conditions or ground water presence. Essentially it is a wall in the way that it is constructed but a sleeper in the way that it functions. .
Normal construction is of brick or concrete block either in stretcher bond or header stretcher bond
References
Sleeper wall Wikipedia(Text) CC BY-SA