Slow-wave sleep ( SWS), often referred to as deep sleep, consists of stage 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales (R & K) standard of 1968. As of 2008, the American Academy of Sleep Medicine (AASM) has discontinued the use of stage 4, such that the previous stages 3 and 4 now are combined as stage 3.