What is "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 is now 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.

Slow-wave sleep is considered important to consolidate new memories. This is sometimes referred to as "sleep-dependent memory processing". Impaired memory consolidation has been effected in individuals with primary insomnia who thus do not perform as well as normal patients in memory tasks following a period of sleep. Furthermore, slow-wave sleep improves declarative memory, which is the fact-based, or episodic, memory. A central model has been hypothesized that the long-term memory storage is facilitated by an interaction between the hippocampal and neocortical networks. In several studies, after the subjects have had training to learn a declarative memory task, the density of human sleep spindles was significantly higher compared to the non-learning control task. This is the result of the spontaneously occurring wave oscillations that account for the intracellular recordings from thalamic and cortical neurons.

Sleep deprivation studies with humans suggest that the primary function of slow-wave sleep may be to allow the brain to recover from its daily activities. Glucose metabolism in the brain increases as a result of tasks that demand mental activity. Another function affected by slow-wave sleep is the secretion of growth hormone, which is always greatest during this stage. It is also thought to be responsible for a decrease in sympathetic and increase in parasympathetic neural activity.