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Slow-wave sleep is considered important for memory consolidation.[5] This is sometimes referred to as "sleep-dependent memory processing".[6] 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.[7][8] Furthermore, slow-wave sleep improves declarative memory (which includes semantic and 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.[7] In several studies, after the subjects have had 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.[9][10] This associated with the spontaneously occurring wave oscillations that account for the intracellular recordings from thalamic and cortical neurons.[11]
A memory reactivation experiment during SWS was conducted using odor as a cue, given that it does not disturb ongoing sleep, over a prior learning task and sleep sessions. The region of the hippocampus was activated in response to odor re-exposure during SWS. This stage of sleep has an exclusive role as a context cue that reactivates the memories and favors their consolidation.[12] A further study demonstrated that when subjects heard sounds associated with previously shown pictures-locations, the reactivation of individual memory representations was significantly higher during SWS (compared to other sleep stages).[14]
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.[14]
Acetylcholine plays an essential role in hippocampus-dependent memory consolidation. An increased level of cholinergic activity during SWS is known to be disruptive for 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 in order to consolidate sleep-related declarative memory.[15]
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.[4] Another function affected by slow-wave sleep is the secretion of growth hormone, which is always greatest during this stage.[16] It is also thought to be responsible for a decrease in sympathetic and increase in parasympathetic neural activity.[16]
Prior to 2007, the American Academy of Sleep Medicine (AASM) divided slow-wave sleep into stages 3 and 4.[17][18][19] The two stages are now combined as "Stage three" or N3. An epoch (30 seconds of sleep) which consists of 20% or more slow-wave (delta) sleep is now considered to be stage three.[____________________]
Longer periods of SWS occur in the first part of the night, primarily in the first two sleep cycles (roughly three hours). Children and young adults will have more total SWS in a night than older adults. The elderly may not go into SWS at all during many nights of sleep.[1_______________]
Slow-wave sleep is necessary for survival. Some animals, such as dolphins and birds, have the ability to sleep with only one hemisphere of the brain, leaving the other hemisphere awake to carry out normal functions and to remain alert. This kind of sleep is called unihemispheric slow-wave sleep, and is also partially observable in human beings. Indeed, a study reported a unilateral activation of the somatosensorial cortex when a vibrating stimulus was put on the hand of human subjects. The recordings show an important inter-hemispheric change during the first hour of non-REM sleep and consequently the presence of a local and use-dependent aspect of sleep.[23] Another experiment detected a greater number of delta waves in the frontal and central regions of the right hemisphere.[24]
Furthermore, a faster behavioral reactivity is detected in the left hemisphere during SWS of the first night. The rapid awakening is correlated to the regional asymmetry in the activities of SWS. These findings show that the hemispheric asymmetry in SWS plays a role as a protective mechanism. SWS is therefore sensitive to danger and non-familiar environment, creating a need for vigilance and reactivity during sleep.[25]
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 hormone is also secreted during this stage, which leads some scientists to hypothesize that a function of slow wave sleep is to facilitate the healing of muscles as well as repair damage to tissues.[26][27] Lastly, glial cells within the brain are restored with sugars to provide energy for the brain.[28]
Learning and memory formation occurs during wakefulness by the process of long-term potentiation; SWS is associated with the regulation of synapses thus potentiated. SWS has been found to be involved in the downscaling of synapses, in which strongly stimulated or potentiated synapses are kept while weakly potentiated synapses either diminish or are removed.[29] This may be helpful for recalibrating synapses for the next potentiation during wakefulness and for maintaining synaptic plasticity. Notably, new evidence is showing that reactivation and rescaling may be co-occurring during sleep.[30]
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 that 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.[31]
The accumulation of amyloid beta (A) in the prefrontal cortex is associated with the disruption or reduction of slow waves of NREM sleep. Therefore, this may reduce the ability for memory consolidation in older adults.[32]
Though SWS is fairly consistent within the individual, it can vary across individuals. Age and sex 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, so 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.[33]
Mental disorders play a role in individual differences in the quality and quantity of SWS: subjects with depression show a lower amplitude of slow-wave activity (SWA) compared to healthy participants. Sex differences also persist in the former group: depressed men present significantly lower SWA amplitude. This sex divergence is twice as large as the one observed in healthy subjects. However, no age-related difference concerning SWS can be observed in the depressed group.[34]
The chemical gamma-hydroxybutyric acid (GHB) has been studied to increase SWS.[44][45] In the United States, the Food and Drug Administration (FDA) permits the use of GHB under the trade name Xyrem to reduce cataplexy attacks and excessive daytime sleepiness in patients with narcolepsy.
Jump into bed, press play and drift away to deep and restful sleep. Hosted by Karissa Vacker, an award-winning voice and meditation guide, bringing you an original episode every week. Sleep Wave has been designed with love, to help you relax and fall asleep quickly and peacefully each night, so you can wake up happy. 5376163bf9
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