Furthermore, slow-wave sleep improves declarative memory (which includes semantic and episodic memory). Impaired memory consolidation has been seen in individuals with primary insomnia, who thus do not perform as well as those who are healthy in memory tasks following a period of sleep. This is sometimes referred to as 'sleep-dependent memory processing'. Slow-wave sleep is considered important for memory consolidation. 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. The second section of the wave signifies an 'up state', an excitation or depolarizing phase in which the neurons fire briefly at a high rate. This is the period when the neocortical neurons are able to rest. The first section of the wave signifies a 'down state', an inhibition or hyperpolarizing phase in which the neurons in the neocortex are silent. This period of sleep is called slow-wave sleep because the EEG activity is synchronized, characterised by slow waves with a frequency range of 0.5–4.5 Hz, relatively high amplitude power with peak-to-peak amplitude greater than 75µV. 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.

It usually lasts between 70 and 90 minutes and takes place during the first hours of the night. Slow-wave sleep ( SWS), often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep. Ostriches sleeping, with REM and slow-wave sleep phases

Introduction to deep sleep stage