<P> Slow - wave sleep (SWS), often referred to as deep sleep, consists of stage three (combined stages 3 and 4) of non-rapid eye movement sleep . Initially, SWS consisted of both Stage 3, which has 20 - 50 percent delta wave activity, and Stage 4, which has more than 50 percent delta wave activity . However, as of 2008, the American Academy of Sleep Medicine (AASM) has discontinued the use of Stage four as a separate stage . Thus, the two stages are now combined as "Stage three" or N3 . An epoch (30 seconds of sleep) which consists of 20% or more slow - wave (delta) sleep is now considered to be stage three . </P> <P> This period of sleep is called slow - wave sleep because the EEG activity is synchronized, producing slow waves with a frequency range of 0.5 - 2 Hz and peak - to - peak amplitude greater than 75μV . The first section of the wave signifies a "down state," which is an inhibition or hyperpolarizing phase in which the neurons in the neocortex are silent . This is the period when the neocortical neurons are able to rest . The second section of the wave signifies an "up state," which is an excitation or depolarizing phase in which the neurons fire briefly at a high rate . The principal characteristics during slow - wave sleep that contrast with REM sleep are moderate muscle tone, slow or absent eye movement, and lack of genital activity . </P>

Which stage of sleep is represented by delta waves