Sleep is a function present in all species across the globe. It is known to benefit cognitive function and enhance learning and memory in both humans and animals. While not yet completely understood it is an essential part of life for the human body. Sleep deprivation can have negative effects on the alertness and cognitive performance in the brain leading to decreased activity and function. The longer an individual stays awake, the more the homeostatic functions of the brain are negatively affected. Emotion, memory, and deductive abilities can all be impaired when a person doesn’t receive an adequate amount of sleep. To combat this the brain functions on different sleep/wake cycles releases hormones to the body in waves to either coerce the body to sleep or to keep it awake. Once asleep the brain goes through sleep cycles in order to prepare the body for deep sleep. During this period of REM (Rapid Eye Movement) sleep the brain kickstarts brain activity to enhance learning and memory ability through neuronal communication. The brain needs to have a constant cycle of sleep in order to maintain optimal functionality and usability. To truly evaluate the effects of sleep deprivation; studies have been completed to test the varying effects that the amount of sleep that a human being receives daily and the period of time that an individual goes without sleep can have on the human mind. It has been determined that sleep cannot be chronically reduced in an individual without varying detrimental effects taking place on the mind and body.
The human brain is one of the most complex living structures in the world. It works as a command center for the body, processing information that it receives from different senses and areas of the body and then sending messages back down in response. The brain is split into two halves which are connected to each other and the rest of the body by a thick bundle of nerve fibers, called the corpus callosum (Informed 2009). These two hemispheres are made up of four different lobes; all controlling different functions in the body. The frontal lobe is associated with reasoning, planning, parts of speech, movement, emotions, and problem solving. The parietal lobe is associated with movement, orientation, recognition, and perception of stimuli. The occipital lobe is associated with visual processing. And the temporal lobe is associated with perception and recognition of auditory stimuli, memory, and speech (Queensland 2018). Because of all these connections from the brain to the rest of the body any sort of damage to any area could have widespread effects on the body as a whole. Due to this overall complexity it has been found that certain factors such as sleep can have varying effects on how the brain functions and of how it communicates with the rest of the body.
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Sleep is a passive, dormant part of a person’s daily life. It is involved in physiological processes in the brain and periphery, such as temperature and hormone regulation, neuromodulation, brain activity, and gene expression (Johanna and Julie 2019). A persons quality of life can be heavily disrupted if they do not get an adequate amount of sleep for what their body requires. A lack of adequate sleep, or sleep deprivation, reduces workplace productivity, public safety, and personal well being (Thomas et al 2000). Individuals will normally enter periods of sleep deprivation for reasons such as deadlines and occupational requirements. Studies of sleep deprivation typically examine the effects of total sleep deprivation, defined as 24-hours or more without sleep, though repeated nights of partial sleep deprivation have similarly detrimental effects (H.P.A et al 2003).
Two of the most notable features of sleep deprivation are a diminished attention span and cognitive performance. In behavioral studies, sleep deprivation is associated with deficits on a range of tasks requiring selective attention, including filtering irrelevant visual stimuli from a memory array, finding embedded figures in complex images, and performance on Stroop-like tasks (the difficulty to name the ink color of a color word if there is a mismatch between ink color and word) (Eve et al 1999). Similarly, complex task performance is impaired, as reflected by tests of working memory, verbal fluency and speech articulation, language, logical reasoning, creative and flexible thinking and planning, decision making, and judgment (Harrison and Horn 1999). These affects can take place within the first 24 hours without sleep and get gradually worse as time continues.
During sleep, the average person passes through four stages: stages 1, 2, 3, and REM (rapid eye movement) sleep. Studies emphasize that the four long‐term memory systems (procedural memory, perceptual representation system, semantic and episodic memory) benefit heavily from REM sleep (Rauchs et al 2005). REM sleep is also the time a person experiences the most intense dreams, because this is the time that the brain is considered to be most active. REM sleep deprivation has been found to have a significant effect on emotional neural excitability necessary for evaluation of potential danger and for controlling reactivity to emotional, particularly threat-related stimuli (Alejandra et al 2012). Without this area or stage of sleep the human mind would slowly cave into mental impairment, dementia, and insanity.
The brain is continually studied in hopes of one day truly understanding it. Sleep study is one of the best avenues for mapping the brain in order to determine its full potential. Through the study of the effects of sleep deprivation on the brain we can learn about how our brain, and in-turn our bodies, react to the withdrawal of sleep over varying amounts of time.
Sleep deprivation is now considered to be a widespread problem across the globe. Nearly 100,000 deaths every year is caused by drivers who are suffering from fatigue. In the on the move lifestyle that our society has developed it’s pretty obvious to see why the average amount of sleep has dropped from 8 hours to 6.8. Since each individual is responsible for his/her own lifestyle, its next to impossible to put up a law regulating the amount of sleep a person needs. Therefore, sleep deficiency will remain a prominent pain in the side of individuals everywhere unless they, as an individual, decides to better their sleep lifestyle. The goal of this review will be to determine why the brain requires a set amount of sleep and how sleep deprivation has an effect on cognitive functions such as attention and working memory.
The Background of Sleep
Every human being on the planet requires a different amount of sleep in order to function properly. The average sleep length is between 7 and 8.5 hours per day. Sleep is considered to be important to body restitution, energy conservation, thermoregulation, and tissue recovery. In addition, sleep is essential for cognitive performance, especially memory consolidation (Maquet 2001). Sleep is regulated by two processes: a homeostatic process (S) and circadian process (C) (Achermann 2004). The homeostatic process is the bodies attempt to maintain a constant and balanced internal environment. This depends on sleep and wakefulness, which are influenced by different neurotransmitter signals in the brain like: histamine, dopamine, norepinephrine, serotonin, glutamate, orexin and acetylcholine. The longer a person stays awake the more the brain tells them that they need sleep. The circadian process is like a 24-hour internal clock that is constantly running in the background of the brain and cycles between sleepiness and alertness at regular intervals. When the alerting areas of the brain are most active, they send arousal signals to the cerebral cortex, while at the same time inhibiting activity in the area of the brain responsible for promoting sleep, resulting in a period of stable wakefulness. When the sleep-promoting areas of the brain are most active they inhibit activity in areas of the brain responsible for promoting wakefulness, resulting in a period of stable sleep. The neurotransmitters: histamine, dopamine, norepinephrine, serotonin, glutamate, orexin and acetylcholine are all imperative in driving wakefulness and sleep. To initiate the beginning of sleep the brain sends signals to the body to release melatonin, which is used to increase drowsiness in the body. After sleep as your body prepares to wake it releases cortisol which is a hormone that naturally prepares your body to wake up The interaction of these two processes with each other determines the sleep/wake cycle of the body and can be used to describe fluctuations in alertness and vigilance that a person receives on a daily basis.
What is Sleep Deprivation and General Effects
Sleep deprivation is one of the largest problems related to sleep and the brain. According to UT Medical Center, 50 million to 70 million U.S. adults have a sleep disorder with almost 1 in 3 experiencing short-term insomnia and about 1 in 10 suffering from chronic insomnia, otherwise known as the inability to go to sleep and/or stay asleep. It is linked to diminished awareness, cognitive impairments, irritability, delayed responses, and wake-state instability. The effects of sleep deprivation on alertness and cognitive performance suggest that sleep deprivation interferes with certain areas and pathways of the brain that depend on the prefrontal cortex. These include higher functions, such as language, executive functions, divergent thinking, and creativity (Paula and Paivi 2007). The experimental studies done on sleep deprivation normally differ in terms of the length of sleep, the timing, and the structure of the sleep; with most individuals studied having gone without sleep for about 24-72 hours. This is primarily because the longer an individual goes without sleep the more the body tries to compensate for the lack of brain power and the easier it is to monitor the increase in brain activity by scans such as MRI’s. We assume that individual differences are also important to how people react to sleep deprivation, as some individuals may be more susceptible to the effects of sleep loss than others. It is noted how most older individuals require less sleep or are more tolerable of sleep deficiency than most younger individuals. This is thought to happen because the circadian rhythm is not as strong in older individuals. Similarly, its noted that in terms of cognitive performance, women may endure prolonged wakefulness better than men, whereas physiologically they recover slower (Paula and Paivi 2007).
How Sleep Deprivation Affects the Brain
The most widely studied domains in sleep deprivation research are on attention and working memory. Working memory can be divided into four subsystems: phonological loop, visuospatial sketchpad, episodic buffer and central executive. The phonological loop is assumed to temporarily store verbal and acoustic information (echo memory); the sketchpad, to hold visuospatial information (iconic memory), and the episodic buffer to integrate information from several different sources. The central executive controls them all. Executive processes of working memory play a role in certain attentional functions, such as sustained attention or vigilance (Paula and Paivi 2007). Any time we are sleep deprived our vigilance is extremely impaired, which as a result has an effect on our overall ability to pay attention to even the most trivial of tasks and also distorts our ability to have a working memory. This is due to the lack of sleep slowing the ability of our neural pathways to properly send synaptic signals around the body.
Sleep deprivation makes it difficult for brain cells to communicate effectively which can in turn cause mental lapses that affect memory and visual perception (Samantha 2017). Nerve-signaling chemicals called neurotransmitters control when we are asleep or awake by acting on different groups of neurons in the brain. Neurotransmitters such as serotonin and norepinephrine are some of the known transmitters that keep parts of the brain active while we sleep while others at the base of the brain can signal neurons to switch off the signals which keep us awake (What is 2016). Antidepressant activity of sleep deprivation may be due to an enhancement of serotonergic and/or noradrenergic neurotransmission in the brain. When sleep starts to become drastic the brain begins to actively shut down certain sections that are not necessary for survival and increase brain activity in core areas, affecting multiple parts of the body. This can be seen in observation of MRI scans of sleep deprived individuals where the mid brain area where the thalamus and hippocampus are will increase in output power to try and compensate. The amygdala of the brain, or the part that controls your emotions becomes overactive resulting in heightened and less controllable emotions. Neurons in the brain begin to lose the ability to encode information and translate visual input into conscious thought, thereby slowing down the brains ability to see and react.
Studies on the Effects of Sleep Deprivation
Studies have shown that individuals who had sleep periods between four and six hours for fourteen days had the same memory and cognitive performance of sleep deprivation as an individual who has not slept for two to three days. Chronic restriction of sleep to 4 h and 6 h initially elevated subjective sleepiness ratings on both the Stanford Sleepiness Scale and the Karolinska Sleepiness Scale, but as sleep restriction continued, there were only minor further increases in these ratings. Surprisingly, by the end of the 14 days of sleep restriction, when performance was at its worst levels, subjects in the 4 h and 6 h sleep period conditions reported feeling only slightly sleepy. Therefore, unlike performance measures, sleepiness ratings appeared to show adaptation to chronic partial sleep deprivation (Van Dongen 2003). When following one night of sleep deprivation we see our brain activity, alertness, and cognitive performance impaired greatly; suggesting that the neurobehavioral function of sleep in humans is to restore and sustain normal waking brain activity and behavior. The cortical association findings of the brain when faced with lack of sleep are complementary to studies of slow wave and REM sleep; demonstrating deactivation of cortical regions, with the implication that the need for recuperation during sleep may be greater in these areas relative to other brain regions (Thomas 2000)
The Sleep Cycles
To properly rest the brain a person needs to go through the various sleep cycles. A full sleep cycle starts out in light sleep and progresses through to deep sleep, then reverses back from deep to light sleep before entering REM (Rapid Eye Movement Sleep). You cycle through each of these stages four to five times during the night. Each of the 5 stages of sleep are important but REM sleep is considered to be the most important. During the initial stages of sleep (stage 1 and 2), biological processes in your body will slow down but the brain remains active as it begins the process of deciding which memories to store and which to discard. In the deeper sleep stages (stage 3 and 4) you enter into a near coma-like state, during which physiological cleansing and detoxification processes in the brain take place. Once in REM sleep the eyes will dart back and forth quickly under the eyelids and the brain will become stimulated allowing for increased mental fortitude and memory formation. This is also the stage at which most dreams are created. In this phase, your brain is as active as it is during wakefulness, but your body is paralyzed, which will prevent a person from acting out their dreams (Rauchs 2005). REM sleep occurs at regular intervals during the sleep cycle and occurs during at least one-third of the night. It is important because it exercises important neural connections which are important to mental and overall wellbeing and health. REM sleep deprivation has been found to have a significant effect on emotional neural excitability necessary for evaluation of potential danger and for controlling reactivity to emotional, particularly threat-related stimuli (Alejandra et al 2012). Sleep disorders, alcohol, caffeine, and opioids are known to have a profound effect on the sleep cycle and can lead to fragmented periods of REM sleep. In turn because an individual does not receive the proper brain stimulation needed during the night, they will show symptoms similar to that of sleep deprivation.
Effects on Attentiveness
Sleep deprivation diminishes the ability of the brain to focus and concentrate. A persons performance for attentional tasks will deteriorate in a dose dependent manner based on the amount of accumulated awake time. This will result in unstable task performance where attentional maintenance will become highly variable and erratic (usually causing attention to be sustained, lost, reestablished, then lost again). Sleep loss decreases the ability to perform task-related activities in the frontal and parietal regions of the brain, and also diminishes activity in the visual cortex during visuospatial (hand-eye required) attention tasks (causing the inability to focus on task related jobs for long periods of time) (Alkadhi 2013).
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When in a sleep-rested state, there is equal inhibition of task-related frontoparietal network (FPN) activity and the default mode network (DMN) activity, which is supported by sustained ascending arousal input from the thalamus. Because of this equal inhibition there is consistent attentional and working-memory performance in the brain (Figure 1). The FPN are the brain regions and networks associated with attention and working memory while the DMN are a collection of brain areas that will disengage and re-engage depending on an individual’s performance of externally driven or goal-directed tasks (Krause 2017).
Comparatively when an individual is in a sleep-deprived state, there will be non-equal inhibition between task-related FPN activity and DMN activity, and erratic ascending arousal activity influencing thalamic activity. As a result, there is reduced task related FPN activity and intermittent disturbances of DMN activity during task engagement (Figure 1). Suppression of the DMN is necessary to mobilize appropriate on-task brain networks to achieve successful goal-directed behavior and without its suppression an individual will not be able to hold attention to specific tasks (Krause 2017). When FPN activity and DMN activity are varied it leads to instability in concentration and attention tasks. Once sleep is attained the arousal input from the thalamus will steady allowing equal activity of the FPN and DMN to be reestablished.
Figure 1: A) Shows how brain regions and networks associated with attention and working memory (FPN; red), arousal (thalamus; green) and the default mode network (DMN; blue) are affected by sleep deprivation. B) Shows how constant and erratic ascending arousal activity influencing thalamic activity will have either a stable or unstable equal inhibition between task-related FPN activity and DMN activity (Krause 2017).
Effect on Working Memory
Sleep is critical for a person’s memory. Sleep deprivation impairs learning and the ability to create new memories. Poor sleep diminishes a person’s capacity to recall memories they’ve already made or are going to make. All phases of memory are complex and involve multiple areas of the brain that are affected by lack of sleep. For something to become a memory three functions must occur: acquisition, consolidation, and recall. Acquisition involves learning or experiencing something new, consolidation requires the memory to become stable in the brain, and recall is the ability to access the memory in the future. Both memory acquisition and memory recall take place when you’re awake and the consolidation stage takes place during sleep. Memory consolidation is the brain’s process of storing new memories for long-term retrieval. Without adequate sleep, the brain has a harder time absorbing and recalling new information.
As with attention tasks, fluctuations in thalamic activity and inappropriate persistence of DMN activity are observed during working-memory task performance under sleep deprived conditions. Sleep deprivation will disrupt multiple signaling pathways in the hippocampus that in turn lead to plasticity and memory impairments. The hippocampus acts in a broad network of anatomically and functionally connected cortical regions to provide constant connection and signaling to the rest of the brain (Boonstra 2007).
Sleep deprivation alters learning-related hippocampal connectivity. Functional coupling between the hippocampus and perceptual regions during visual episodic-memory encoding is decreased under sleep deprived conditions. In contrast, sleep deprivation is also associated with increases in hippocampal connectivity with subcortical arousal regions, which include the brainstem and the thalamus (Figure 2). This is thought to be a compensatory effort by the brain to mobilize the basic arousal networks at the sacrifice of cognitive performance (Krause 2017).
Through MRI scans it was found that the average sleep-deprived person’s brain needed to exert more effort than the brain of an average non-sleep deprived person when performing specific tasks (Figure 3). We can deduct from these results that the brains of sleep-deprived people are trying to compensate for the adverse effects created by sleep deprivation.
Figure 2: Sleep deprivation decreases encoding-related activity within the hippocampus (light blue). In sleep-deprived individuals, hippocampal connectivity during encoding is lower than in sleep-rested individuals. In contrast, hippocampal encoding-related connectivity with subcortical arousal regions of the thalamus and brainstem (red) after sleep deprivation is increased (Krause 2017).
Figure 3: In sleep deprived subjects, increased activity was found in certain areas of the prefrontal cortex of the brain, which is where memory formation and logical reasoning take place.
Sleep is an important part of a healthy lifestyle. The brain is constantly releasing neurological hormones to keep the body in a constant cycle of sleep and wake. When a person cannot get the necessary amount of sleep to function correctly, they go into a state of sleep deprivation. The negative effects of sleep deprivation can be seen on cognitive ability, attention, and working memory. All neurological pathways become slowed causing a decreased mental state and slowed ability time. Every system in the body goes into a state of life-support in order to hold out until it can get the brain re-stimulated. Fluctuations in thalamic activity, inappropriate persistence of DMN activity, and decreased hippocampal activity will cause unequal stimulation in the brain, which in turn causes some portions of the brain to shut off in an attempt to increase output in the central areas of the brain to make up for the loss of input brought on by sleep deprivation. As a result, decreased attentiveness and working memory formation will follow. The brain will continually shut down areas of the brain as sleep deprivation continues to try and compensate energy output near the mid brain and brainstem in order to keep basic functions running. When getting the recommended amount of sleep for the certain body type, the brain will flourish with increased mental activity and response time. Data shows that regardless of the of your body; if you don’t receive enough sleep daily or go without sleep for multiple days your body’s systems begin to gradually shut down. The human brain and body need to successfully go through all the stages of the sleep cycle in order to get a good night’s sleep. Achieving REM sleep during slumber will activate neurons in the brain to increase stimulation causing dreaming and refreshed mental fortitude. So, in order to keep the mind and body healthy a full, restful amount of sleep is needed to maximize brain function.
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