Sleep plays a crucial role in brain function and the systematic physiology of many-body systems. Difficulty with sleep has become widely prevalent, consisting of deficits in quantity and quality of sleep. Insomnia is a subtype, associated with difficulty falling asleep, waking up often during the night and having trouble going back to sleep. These sleep difficulties begin with short term sleep disturbances, but can eventually branch to sleep deprivation, which are the same symptoms on a long-term scale. Recent literature has shown a direct relationship with these symptoms and adverse effects on cognitive function. Cognition is a broad term, encompassing a variety of mental processes such as memory, problem-solving, language, planning and attention (Miller et al., 2014). Cognitive impairment is a measurable decline to these cognitive abilities. When determining the fine line between these two key terms, research points to loss of sleep, causing harm to the proper neural and physical function of an individual. This paper will evaluate the potential impact of this neurological basis of the relationship between sleep disturbance and cognitive function, as well as how sleep disturbance can negatively and positively affect an individual’s cognitive function. The related notion that sleep disturbances, or rather the long-term consequence of sleep deprivation on cognitive decline is an ongoing debate; however, the definitive correlation remains unknown.
Sleep insomnia is a primary cause of sleep disturbance. It develops from stress at home or work, poor sleeping habits, overeating and existing mental health disorders, and as a result, cause substantial short- and long-term health conditions. A literature search reported that sleep disturbance correlates with responses such as increased activity in the sympathetic nervous system and hypothalamic–pituitary–adrenal axis, metabolic effects, changes in circadian rhythms, and proinflammatory responses (Mysliwiec et al., 2014). When evaluating healthy adults, short term consequences include increased stress responsivity, somatic pain, reduced quality of life, emotional distress, mood disorders, and more importantly, cognitive, memory and performance deficits. (Medic, Wille & Michiel, 2019). Fatigue is also a significant symptom, which causes a significant deterioration in cognitive and psychomotor skills. (Kahol, Leyba, Deka and Mayes, 2007). Symptoms of cognitive impairment, induced by sleep disturbance can include memory loss, repetition, struggle to recognise familiar people, places and objects, inappropriate decision making, erratic changes in mood or behaviour, speech problems and difficulty in planning and executing daily tasks. Cognitive decline does not follow a regular timeline, and therefore it is often called delirium, which in extreme cases can foreshadow severe medical problems. The cause of cognitive decline is not simple either, considering it is not caused by an underlying disease or condition, nor specific to an age group. For example, a combination of conditions such as dementia, stroke, traumatic brain injury and other developmental disabilities can increase the risk of cognitive decline.
The decrease and irregularity of neural activity due to lack of restoration
There has been substantial research on the negative impact the symptoms of insomnia, have on an individual’s executive function and reaction time. Ultimately, such results have also demonstrated an inability to focus or concentrate on a specific task, causing underlying incompetence for successful memory development. Sleep allows for the strengthening of nerve connections for memory formation (Verceles, 2015). Accordingly, one can see the strength of this literature when examining whole-brain activation results, indicating significant interaction in the right lateral frontal gyrus (rIFG) region. Studies further suggest that the role of rIFG is critical for inhibiting response trends (Aron, Robbins & Poldrack, 2014) and is related to both response and attentional control (Dodds, Morein-Zamor & Robbins, 2011). The rIFG is known as a 'brake', and it can be initiated to both suppress or pause a response when it becomes unnecessary or needs to be refined. Those who showed symptoms of insomnia displayed a decreased integrity in the left thalamus–pars triangularis tracts, which correlated with cognition and daytime sleepiness. These results may imply that insomnia leads to the disintegration of the white-matter tract between the left thalamus and inferior frontal gyrus.
Interestingly, the dorsal and ventral prefrontal cortex (PFC) form-critical regions in the working memory (WM) and executive function systems, that are fundamental to the maintenance and control of information in focus and attention, as well as minimising distraction during thought processing (De Dreu et al., 2012). Suggestive that functional impairment in ventral and dorsal PFC due to short-term sleep disturbance could impair divergent thinking by negatively impacting WM and executive function. Although the evidence above highlights PFC as a crucial target, the weakness of such pieces of literature lies in the uncertainty on how this effect would manifest itself. In part, this is because the neural effects of sleep loss have demonstrated variation and context-dependency. On the one hand, researchers have observed a high hemodynamic response profile (bold signal) concerning verbal learning and logical reasoning following sleep disturbance (Jonelis et al., 2012). The interpretation of this demonstrates the brain's compensatory ability to counteract the impairment of normal brain function in the form of increased activity in the right lateral frontal gyrus (rlFG). While, evidence from working memory (WM) tasks, have demonstrated that sleep disturbance can lead to a reduction in the bold response in PFC (Chee & Chuah, 2008). This variability observed may be a function of task difficulty. For example, the cerebral compensatory response is more likely to be observed in tasks of great difficulty (Drummon et al., 2004). Additionally, research on the neuroscience of sleep loss has alluded that fluid intelligence is also a cause to individual variation when examining the vulnerability of thought processing and creativity abilities and also displayed variation in brain activation between ventral and dorsal PFC. Therefore, even short-term sleep disturbance leads to irregular changes occurring inside the body, which can create an off-balance, causing cognitive decline.
The inability to enhance thought processes to generate new solutions to new stimuli
Furthermore, the literature on short term sleep disturbance has demonstrated that individuals were unable to devise new solutions to new problems, but instead tried to use solutions of the previous problems, during a problem-solving task. To contrary belief, this effect was not driven by the participants’ loss of motivation or interest in the task, but rather, they were incapable of diverting their fixation on previously successful strategies (Horne, 1988 p. 535). In other words, sleep disturbance affected cognitive performance in the form of perseveration, a 'difficulty in changing strategies.' In neural terms, although engagement in thought processing activates a distributed network in the brain, functional magnetic resonance imaging (fMRI) have shown most consistently less activation of the ventral and dorsal aspects of PFC (Kröger et al., 2012). In this sense, the impact of lack of sleep has caused vulnerability to the prefrontal cortex (Jones & Harrison, 2001), which can ultimately result to 'a reversible functional lesion in the PFC' (Lim & Dinges, 2010, p. 376), when considering long term sleep deprivation. Consequently, individuals become restricted to previous solutions, unable to focalise their attention to new stimuli, which is already a negative effect on their cognitive function.
Short term sleep disturbance does not affect a surgeon's skills during night shifts
The literature on the positive impact of sleep disturbance seems to draw a fine line between sleep disturbance and a decrease in cognitive function. Several studies have demonstrated that surgeons' work quality and skills have not diminished who experience sleep disturbances while on overnight shifts. Although one can say, acute call-associated fatigue seems to be a predominantly subjective perception. Physiological factors seem to outbalance an anticipated fatigue-associated impairment of technical performances within low-fidelity VR-tasks. In surgical residents, acute partial sleep disturbance seems to have a positive short-term effect on cognitive skills, leading to enhanced technical performance and increased physical alertness within complex tasks (Schlosser, 2012).
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Further study has exhibited that sleep-deprived residents did not show any performance deficits on the Paced Auditory Serial Addition Test. Cognitive performance remained with no association with sleepiness measures (Reimann et al. 2009). Although there is no doubt that sleep deprivation as a long-term symptom, ultimately impairs human functioning, while typical surgical skills do not necessarily deteriorate with a limited amount of sleep disturbance when under clinical conditions (Lehmann et al. 2010). Hence, when put under constraining conditions, the cognitive function is not impacted, relative to the level of tiredness or sleepiness an individual may be presenting. For example, Amirian et al. (2014), reported that sleep-deprived surgeons did not show any impairment of psychomotor or cognitive performance during a 17-hour night shift. A conclusion from this literature, although not solidified, is that sleep disturbance seems to cause compensatory neural activation for the lack of function from no rest, which allows an individual to remain alert and more focused on detail.
The debate
Nevertheless, there is conflicting evidence regarding the relationship between sleep deprivation, rather than short term sleep disturbance and an individual’s cognitive function. Sleep disorders are common in aging and further increase with advancing age (Gadie et al., 2017). A meta-analysis concluded that persons with symptoms of insomnia displayed impairments in cognitive functioning within episodic memory, working memory and executive functioning (Fortier-Brochu et al., 2012). On the other hand, evidence conflicts, as some studies report no association between sleep disturbance and cognition, some even further hypothesise that sleep disturbance may even present better cognitive functioning. As a result, it seems research has shown a negative correlation between sleep disturbances and cognition; however, the question of the relationship between sleep disturbance and cognitive decline remains undefined.
Mechanisms linking sleep to poor cognitive outcomes or AD pathology
Multiple instruments account for the observed relationship of poor sleep with cognitive decline and AD pathology. Recently, studies are suggestive that sleep loss leads to neuronal damage that could mediate cognitive decline. Cross et al. (2013) employed proton magnetic resonance spectroscopy (MRS), a neuroimaging technique that measures brain metabolites, in the aim of examining cross-sectional network between the Pittsburgh Sleep Quality Index (PSQI) and N-acetyl-aspartate (NAA; an index of neuronal health) and myo-inositol (mI; a measure of gliosis/neuronal injury) in the hippocampus and thalamus of healthy older adults (Cross et al., 2013). This established that more mediocre sleep quality was associated with higher mI in the hippocampus, suggesting that sleep loss may adversely affect this structure, which is central to memory. Conversely, brain aging may negatively affect cognition by undermining slow-wave sleep (SWS). Mander et al. (2013) proposed, the medial prefrontal cortex atrophy in the older population, may contribute to cognitive decline by limiting SWS and subsequently interfering with sleep-dependent memory consolidation (Mander et al., 2013), all part of a disruption to cognitive function.
Additionally, when examining the link between sleep loss to amyloid-beta (Aβ), Xie et al. showed that SWS increases Aβ clearance from the brain and that sleep loss may, as a result, enhance Aβ aggregation (Xie et al., 2013). To further explain these changes, recent findings showed that wakefulness increases synaptic strength, in which sleep then serves to downregulate (Tononi, 2009). They further referred to 'in vivo' and 'in vitro' studies exhibiting the increase of neural activity directly influenced increase in Aβ production (Bero et al., 2011), and argued that sleep loss might also alter Aβ levels through wake-related increases in synaptic strength. However, there are limitations to this study, and further research is needed to evaluate these mechanisms. Furthermore, sleep-disordered breathing (SDB) causes hypoxemia and sleep fragmentation, both of which may lead to cognitive decline and Aβ deposition. SDB interferes with sleep-dependent memory consolidation, which may mediate its effect on cognition (Djonlagic et al., 2014). Likewise, by limiting slow-wave sleep (SWS), sleep-disordered breathing (SDB) may also contribute to Aβ deposition through the mechanisms described above. SDB and sleep loss both are associated with increases in inflammation, which mediates the link between sleep disturbance and both cognitive decline and AD biomarkers (Blackwell et al., 2014).
Impact of the night shift on laparoscopic skills and cognitive function on gynaecologists
A study conducted to assess whether gynaecologists have impaired laparoscopic skills and reduced cognitive function after disrupted sleep, showed there was no evident impairment on their surgical performance. A VR procedural module was completed immediately after a 17.5h on call, which measured several parameters of performance, giving this study satisfactory reliability. Although the realism of the EP module can be debated, the transfer of VR skills to the operation room is generally well established and gynaecologists, showed normal cognitive function, although there were reports of an increase in reaction time. Another limitation is the lack of prospective studies. However, retrospective studies regarding post night-time surgical procedures have not found higher complication rates among patients operated during daytime by surgeons who had performed emergency procedures the night before. There was no evident deterioration of simulated laparoscopic skills compared with normal baselines. However, simple well-known procedures may promote motivational effort and reduce the negative impact of tiredness. While tedious tasks requiring vigilance become more prone to error when wakefulness is prolonged, but in new situations or emergencies, professionals seem to be able to mobilise additional energy to compensate for the short-effects of tiredness. The results of this study do not indicate impairment of simulated surgical skills or significant deterioration of cognitive function among gynecologists direct after only short-term sleep disturbance. (Veddeng et al., 2014)
The relationship between sleep deprivation and cognition has been speculated for quite some time now. This complex topic has continued to thrive through research instruments such as brain imaging studies. Even though there are limitations to this instrument, there are still many other subjective and objective measures that suggest that there is a strong correlation between lack of sleep and cognitive function. However, even though sleep deprivation causes cognitive decline, in some situations, short term sleep disturbance can cause additional and compensatory neural activity, which advances cognitive function. An increase in knowledge in this area is still needed to strengthen the use of music within society to assist individuals in their everyday life, especially with our society's aging population.
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