Slow-wave sleep

Slow-wave sleep (SWS), often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep, according to the Rechtschaffen & Kales (R & K) standard of 1968. There is not a clear distinction between stages three and four. Stage three has 20-50 percent delta activity, whereas stage four has more than 50 percent. As of 2008, the American Academy of Sleep Medicine (AASM) has discontinued the use of stage four, such that the previous stages three and four now are combined as stage three. An epoch (30 seconds of sleep) which consists of 20% or more slow-wave (delta) sleep, now is considered to be stage three.

This period of sleep is called slow-wave sleep because the EEG activity is synchronized, producing slow waves with a frequency of less than 1 Hz and a relatively high amplitude. The first section of the wave signifies a down state, which is an inhibition period 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 period in which the neurons fire briefly at a high rate. The former state is a hyperpolarizing phase and the latter is a depolarizing phase. 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.