Slow-wave sleep

Slow-wave sleep (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves.

Slow-wave sleep usually lasts between 70 and 90 minutes, taking place during the first hours of the night. Slow-wave sleep is characterised by moderate muscle tone, slow or absent eye movement, and lack of genital activity. Slow-wave sleep is considered important for memory consolidation, declarative memory, and the recovery of the brain from daily activities.

Before 2007, the term slow-wave sleep referred to the third and fourth stages of NREM. Current terminology combined these into a single stage three.

This period of sleep is called slow-wave sleep because the EEG activity is synchronized, and characterised by slow waves with a frequency range of 0.5–4.5 Hz and a relatively high amplitude power with peak-to-peak amplitude greater than 75 μV. 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 is the period when the neocortical neurons can rest. 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. 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.

Before 2007, the American Academy of Sleep Medicine (AASM) divided slow-wave sleep into stages 3 and 4. The two stages are now combined as Stage Three or N3. An epoch (30 seconds of sleep) that consists of 20% or more slow-wave (delta) sleep is now considered slow-wave sleep.

Slow-wave sleep is considered important for memory consolidation. This is sometimes referred to as "sleep-dependent memory processing". 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. Furthermore, slow-wave sleep improves declarative memory (which includes semantic and episodic memory). A central model has been hypothesized that long-term memory storage is facilitated by an interaction between the hippocampal and neocortical networks. In several studies, after the subjects have had the training to learn a declarative memory task, the density of human sleep spindles present was significantly higher than the signals observed during the control tasks, which involved similar visual stimulation and cognitively-demanding tasks but did not require learning. This associated with the spontaneously occurring wave oscillations that account for the intracellular recordings from thalamic and cortical neurons.

Specifically, SWS presents a role in spatial declarative memory. Reactivation of the hippocampus during SWS is detected after the spatial learning task. In addition, a correlation can be observed between the amplitude of hippocampal activity during SWS and the improvement in spatial memory performance, such as route retrieval, on the following day. Additionally, studies have found that when odour cues are given to subjects during sleep, this stage of sleep exclusively allows contextual cues to be reactivated after sleep, favoring their consolidation. A separate study found that when subjects hear sounds associated with previously shown pictures of locations, the reactivation of individual memory representations was significantly higher during SWS as compared to other sleep stages.

Affective representations are generally better remembered during sleep compared to neutral ones. Emotions with negative salience presented as a cue during SWS show better reactivation, and therefore an enhanced consolidation in comparison to neutral memories. The former was predicted by sleep spindles over SWS, which discriminates the memory processes during sleep as well as facilitating emotional memory consolidation. Acetylcholine plays an essential role in hippocampus-dependent memory consolidation. An increased level of cholinergic activity during SWS is known to be disruptive to memory processing. Considering that acetylcholine is a neurotransmitter that modulates the direction of information flow between the hippocampus and neocortex during sleep, its suppression is necessary during SWS to consolidate sleep-related declarative memory.