Slow-wave sleep (SWS), often referred to as deep 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 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.

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 slow-wave sleep affects 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.

Slow wave sleep is the constructive phase of sleep for recuperation of the mind-body system in which it rebuilds itself after each day. Substances that have been ingested into the body while an organism is awake are synthesized into complex proteins of living tissue. Growth hormones are also secreted to facilitate the healing of muscles as well as repairing damage to any tissues. Lastly, glial cells within the brain are restored with sugars to provide energy for the brain.

J. A. Horne (1978) reviewed several experiments with humans and concluded that sleep deprivation has no effects on people’s physiological stress response or ability to perform physical exercise. It did, however, have an effect on cognitive functions. Some people reported distorted perceptions or hallucinations and lack of concentration on mental tasks. Thus, the major role of sleep does not appear to be rest for the body, but rest for the brain.

When sleep-deprived humans sleep normally again, the recovery percentage for each stage of sleep is not the same. Only seven percent of stages one and two are regained, but 68 percent of stage-four slow-wave sleep and 53 percent of REM sleep are regained. This suggests that stage-four sleep (known today as the deepest part of stage-three sleep) is more important than the other stages.

During slow-wave sleep, there is a significant decline in cerebral metabolic rate and cerebral blood flow. The activity falls to about 75 percent of the normal wakefulness level. The regions of the brain that are most active when awake have the highest level of delta waves during slow-wave sleep. This indicates rest is geographical. The “shutting down” of the brain accounts for the grogginess and confusion if someone is awakened during deep sleep since it takes the cerebral cortex time to resume its normal functions.

According to J. Siegel (2005), sleep deprivation results in the build-up of free radicals and superoxides in the brain. Free radicals are oxidizing agents that have one unpaired electron, making them highly reactive. These free radicals interact with electrons of biomolecules and damage cells. In slow-wave sleep, the decreased rate of metabolism reduces the creation of oxygen byproducts, thereby allowing the existing radical species to clear. This is a means of preventing damage to the brain.

Bedwetting, night terrors, and sleep-walking are all common behaviors that can occur during stage three of sleep. These occur most frequently amongst children, who then generally outgrow them. Another problem that may arise is sleep-related eating disorder. An individual will sleep-walk leaving his or her bed in the middle of the night seeking out food, and will eat not having any memory of the event in the morning.Over half of individuals with this disorder become overweight. Sleep-related eating disorder can usually be treated with dopaminergic agonists, or topiramate which is an anti-seizure medication. This nocturnal eating throughout a family suggests that heredity may be a potential cause of this disorder.Though SWS is fairly consistent within the individual, it can vary across individuals. Age and gender have been noted as two of the biggest factors that affect this period of sleep. Aging is inversely proportional to the amount of SWS beginning by midlife and therefore, SWS declines with age. Sex differences have also been found, such that females tend to have higher levels of SWS compared to males, at least up until menopause. There have also been studies that have shown differences between races. The results showed that there was a lower percentage of SWS in African Americans compared to Caucasians, but since there are many influencing factors (e.g. body mass index sleep-disordered breathing, obesity, diabetes, and hypertension) this potential difference must be investigated further