Sleep and memory

In 1801, David Hartley first suggested that dreaming altered the associative planetary links within the brain during rapid eye movement (REM) periods of the sleep cycle. The idea that sleep had a mentally restorative effect, sorting out and consolidating memories and ideas, was intellectually acceptable by the end of the 19th century. In 'Peter and Wendy', J.M. Barrie wrote It is the nightly custom of every good mother after her children are asleep to rummage in their minds and put things straight for next morning, repacking into their proper places the many articles that have wandered during the day....When you wake in the morning, the naughtinesses and evil passions with which you went to bed have been folded up small and placed at the bottom of your mind; and on the top, beautifully aired, are spread out your prettier thoughts, ready for you to put on. The stories of Peter Pan take place in a fictional world and contain many allusions to aspects of cognitive psychology, some of which predate their formal scientific investigation. The first semi-multiple-systematic study of the connection between sleep and memory was conducted in 1924 by Jenkins and Dallenbach, for the purpose of testing Hermann Ebbinghaus' memory decay theory. As behavioral characteristics of the effects of sleep and memory are becoming increasingly understood and supported, researchers are turning to the weakly understood neural basis of sleep and memory. == Sleep cycles ==

Sleep progresses in a cycle which consists of five stages. Four of these stages are collectively referred to as non-rapid eye movement (NREM) sleep whereas the last cycle is a rapid eye movement period. A cycle takes approximately 90–110 minutes to complete. Wakefulness is found through an electroencephalogram (EEG) which is measured and characterized by beta waves, the highest in frequency but lowest in amplitude, and tend to move inconsistently due to the vast amount of stimuli a person encounters while awake. • Pre-sleep is the period of decreased perceptual awareness where brain activity is characterized by alpha waves which are more rhythmic, higher in amplitude and lower in frequency compared to beta waves. • Stage one is characterized by light sleep and lasts roughly 10 minutes. Brain waves gradually transition to theta waves. • Stage two also contains theta waves; however, random short bursts of increased frequency called sleep spindles are a defining characteristic of this stage. • Stage three and four are very similar and together are considered to be "deep sleep". In these stages brain activity transitions to delta waves which are the lowest in frequency and highest in amplitude. These two stages combined are also called slow wave sleep (SWS). • Stage five, REM sleep, is one of the most interesting stages as brain wave patterns are similar to those seen in relaxed wakefulness. This is referred to as "active sleep" and is the period when most dreaming occurs. REM sleep is also thought to play a role in the cognitive development of infants and children as they spend much more of their sleep in REM periods opposed to adults. During the first half of the night, the largest portion of sleep is spent as SWS, but as the night progresses SWS stages decrease in length while REM stages increase. == Memory terms ==

Stabilization vs. enhancement Stabilization of a memory is the anchoring of a memory in place, in which a weak connection is established. Stabilization of procedural memories can even occur during waking hours, suggesting that specific non-declarative tasks are enhanced in the absence of sleep. When memories are said to be enhanced, however, the connection is strengthened by rehearsal as well as connecting it to other related memories thereby making the retrieval more efficient. Whereas stabilization of non-declarative memories can be seen to occur during a wakeful state, enhancement of these sensory and motor memories has most been found to occur during nocturnal sleep. Use-dependent brain activity is a result of the neuronal usage that occurred during the previous waking hours. Essentially it is neuronal regeneration, activity that occurs whether the person has learnt anything new or not. Pre-training vs post-training sleep deprivation Researchers approach the study of sleep and memory from different angles. Some studies measure the effects of sleep deprivation after a novel task is taught (the subject learns the task and is sleep deprived afterwards). This is referred to as post-training sleep deprivation. Conversely, other experiments have been conducted that measure the effects of sleep deprivation before a task has been taught (the subject is sleep-deprived and then learns a task). This is referred to as pre-training sleep deprivation. Offline memory processing This is the processing of memories out of conscious awareness. For example, after someone has been reading a book, their brain continues to process the experience during other activities. This "offline" processing likewise occurs during sleep. == Methods of measuring memory ==

Behavioral measures • A self-ordered pointing task is a task of memory where a participant is presented with a number of images (or words) which are arranged on a display. Several trials are presented, each with a different arrangement and containing some of the previous words or images. The task for the participant is to point to a word or image they had not previously pointed to in other trials. • In a recency discrimination task participants are shown two trials of image presentation and then a third trial containing a mixture of images from the first and second trial. Their task is to determine whether the image was from the most recent presentation or the previous one. • In a mirror tracing task participants are asked to trace several figures as fast and as accurately as possible which they can only see in a mirror. Speed is recorded as well as how much they deviate from the original image (accuracy). • In the reach-to-grasp task rodents learned a skilled forelimb task. Sleep improved movement speed with preservation of accuracy. These offline improvements were linked to both replay of task-related ensembles during non-rapid eye movement (NREM) sleep and temporal shifts that more tightly bound motor cortical ensembles to movements. • In a neuroprosthetic task rodents trained to perform a simple brain–machine interface task in which the activity of a set of motor cortical units was used to control a mechanical arm attached to a feeding spout. After successful learning, task-related units specifically experienced increased locking and coherency to slow-wave activity (SWA) during sleep. The time spent in SWA predicted the performance gains upon awakening. • In a block tapping task participants are asked to type a sequence of five numbers with their dominant or non-dominant hand (specified in experiment), for an allotted period of time, followed by a rest period. A number of these trials occur and the computer records the number of sequences completed to assess speed and the error rate to assess accuracy. Neural imaging measures Neuroimaging can be classified into two categories, both used in varying situations depending on what type of information is needed. Structural imaging deals predominately with the structure of the brain (computed tomography) while functional imaging deals more heavily with metabolic processes in regards to anatomical functioning (positron emission tomography, functional magnetic resonance imaging). In recent years, the relationship between sleep and memory processes had been aided by the development of such neuroimaging techniques. Positron emission tomography (PET) is used in viewing functional processes of the brain (or other body parts). A Positron-emitting radionuclide is injected into the bloodstream and emits gamma rays which are detected by an imaging scanner. Computer analysis then allows for a 3-dimensional reconstruction of the brain region or body part of interest. Functional magnetic resonance imaging (fMRI) is a type of brain imaging that measures the change of oxygen in the blood due to the activity of neurons. The resulting data can be visualized as a picture of the brain with colored representations of activation. Molecular measures Although this may be seen as similar to neuroimaging techniques, molecular measures help to enhance areas of activation that would otherwise be indecipherable to neuroimaging. One such technique that aids in both the temporal and visual resolution of fMRI is the blood-oxygen-level dependent (BOLD) response. Changes in the BOLD response can be seen when there is differing levels of activation in suspected areas of functioning. Energy is supplied to the brain in the form glucose and oxygen (which is transferred by hemoglobin). The blood supply is consistently regulated so that areas of activation receive higher amounts of energy compared to areas that are less activated. In positron emission tomography, the use of radionuclides (isotopes with short half lives) facilitates visual resolution. These radionuclides are attached to glucose, water and ammonia so that easy absorption into the activated brain areas is accomplished. Once these radioactive tracers are injected into the bloodstream, the efficiency and location of chemical processes can be observed using PET. ==Methods of measuring sleep==

Electrophysiological measures The main method of measuring sleep in humans is polysomnography (PSG). For this method, participants often must come into a lab where researchers can use PSG to measure things such as total sleep time, sleep efficiency, wake after sleep onset, and sleep fragmentation. PSG can monitor various body functions including brain activity (electroencephalography), eye movement (electrooculography), muscle movement (electromyography), and heart rhythm (electrocardiography). • Electroencephalography (EEG) is a procedure that records electrical activity along the scalp. This procedure cannot record activity from individual neurons, but instead measures the overall average electrical activity in the brain. • Electrooculography (EOG) measures the difference in electrical potential between the front and the back of the eye. This does not measure a response to individual visual stimuli, but instead measures general eye movement. • Electromyography (EMG) is used to records the electrical activity of skeletal muscles. A device called an electromyograph measures the electrical potential of muscle cells to monitor muscle movement. • Electrocardiography (ECG or EKG) measures the electrical depolarization of the heart muscles using various electrodes placed near the chest and limbs. This measure of depolarization can be used to monitor heart rhythm. Behavioural measures Actigraphy is a common and minimally invasive way to measure sleep architecture. Actigraphy has only one method of recording, movement. This movement can be analyzed using different actigraphic programs. As such, an actigraph can often be worn similarly to a watch, or around the waist as a belt. Because it is minimally invasive and relatively inexpensive, this method allows for recordings outside of a lab setting and for many days at a time. But, actigraphy often over estimates sleep time (de Souza 2003 and Kanady 2011). ==Competing theories==

Most studies point to the specific deficits in declarative memories that form pre or post REM sleep deprivation. Conversely, deficits in non-declarative memory occur pre or post NREM sleep deprivation. This is the stage specific enhancement theory. There is also a proposed dual-step memory hypothesis suggesting that optimal learning occurs when the memory trace is initially processed in SWS and then REM sleep. Support for this is shown in many experiments where memory improvement is greater with either SWS or REM sleep compared to sleep deprivation, but memory is even more accurate when the sleep period contains both SWS and REM sleep. == Declarative memory ==

Declarative memory is the memory for conscious events. There are two types of declarative memory: episodic and semantic. Episodic memory is for remembering experiences or personal facts whereas semantic memory is remembering specific facts. Declarative memory is typically considered to be an explicit memory because the individual must consciously try to remember it. Temporal memory Temporal memory consists of three main categories, although they are still debated by psychologists and neurobiologists; the categories are immediate memory, short-term and long-term memory. Immediate memory is when a memory is recalled based on recently presented information. Short-term memory is what is used when retaining information that had been presented within seconds or minutes prior. A type of short-term memory is known as working memory, which is the ability to retain information that is necessary to carry out sequential actions. Long-term memory is the retention of information for longer periods of time, such as days, weeks or even a lifetime. In a study, participants were placed into four groups: two control groups given either caffeine or a placebo, and two groups that were sleep deprived for 36 hours, given either caffeine or a placebo. The task used to measure temporal memory consisted of discriminating between recent and less recent face presentations. A set of twelve unfamiliar faces were presented sequentially every 10 seconds. A self-ordered pointing task was used afterwards for 5 minutes to prevent rehearsal and to keep tired participants occupied. This required them to mark any new items seen (either nouns or abstract shapes) presented on 12 sheets. A second set was presented, followed by another self-ordered pointing task, and then a random sequence of 48 faces either containing previously presented faces or new ones were shown to the participant. They were asked if they recognized the faces and whether they were from the first or second set. Results indicate that sleep deprivation does not significantly affect recognition of faces, but does produce a significant impairment of temporal memory (discriminating which face belonged to which set). Caffeine was found to have a greater effect on the sleep deprived group as compared to the placebo group deprived of sleep, but still performed worse than both control groups. Sleep deprivation was also found to increase beliefs of being correct, especially if the participants were wrong. Brain imaging studies of those sleep deprived found that the greatest reduction in metabolic rate is in the prefrontal cortex. Verbal learning A blood-oxygen-level dependent (BOLD) fMRI was used in a study by Drummond et al. to measure the brain's response to verbal learning following sleep deprivation. An fMRI recorded brain activity during a verbal learning task of participants either having a normal night of sleep or those deprived of 34.7 (± 1.2) hours of sleep. The task alternated between a baseline condition of determining whether nouns were upper or lower case, and an experimental condition of memorizing a list of nouns. The results of the study indicate that performance is significantly worse on free recall of the list of nouns when sleep deprived (an average of 2.8 ± 2 words) compared to having a normal night of sleep (4.7 ± 4 words). In terms of brain regions activated, the left prefrontal cortex, premotor cortex, and temporal lobes were found to be activated during the task in the rested state, and discrete regions of the prefrontal cortex were even more activated during the task in the sleep deprived state. As well, the bilateral parietal lobe, left middle frontal gyrus, and right inferior frontal gyrus were found to be activated for those sleep deprived. The implication of these findings are that the brain can initially compensate for the effects of sleep deprivation while maintaining partially intact performance, which declines with an increasing time-on-task. This initial compensation may be found in the bilateral regions of both frontal and parietal lobes and the activation of the prefrontal cortex is significantly correlated with sleepiness. Slow wave sleep (SWS) Slow wave sleep (SWS) has often been associated with successful performance in declarative memory recall tasks. For example, declarative and procedural memory recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep). Based on targeted memory reactivation (TMR) that use associated memory cues for triggering memory traces during sleep, recent studies have been reassuring the importance of nocturnal SWS for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people's memory performance at declarative recalls. Increased slow activity and sleep time spent in SWS have been also related to better performance in implicit learning. Macroscopic brain systems The most prominent population pattern in the hippocampus during nREM sleep is called sharp wave ripples (SPW-Rs). SPW-Rs are the most synchronous neuronal patterns in the mammalian brain. As many as 15-30 percent of neurons can fire synchronously in 50-200 ms within the CA3-CA2-CA1 regions, the subicular complex, and the entorhinal cortex during SPW-Rs (as opposed to ~1 percent during active waking and REM sleep). Neurons within SPW-Rs are sequentially organized, and many of these fast sequences relate to the order of neuronal firing during the animal's pre-sleep experience. For example, when a rat explores a maze, place cell sequences in different arms of the maze are replayed in either forward (as during the experience itself) or reverse order, but compressed in time several-fold. SPW-Rs are temporally linked to both sleep spindles and the slow oscillations of the neocortex. Interfering with SPW-Rs or their coupling with neocortical slow oscillations results in memory impairment, which can be as severe as surgically damaging the hippocampus and/or associated structures. SPW-Rs are therefore the most prominent physiological biomarkers of episodic (i.e., hippocampus-dependent) memory consolidation (Buzsáki 2015). Neural forebrain reverberation correlation Researchers used rats in order to investigate the effects of novel tactile objects on the long-term evolution of the major rodent forebrain loops essential in species-specific behaviours, including such structures as the hippocampus, putamen, neocortex and the thalamus. The rats were monitored but not bothered for 48–96 hours, allowing normal wake-sleep cycles to occur. At some point four novel tactile objects were placed in the four corners of the rat's cage. They were all very different from one another and they were there for a total of one hour. The brain activity during this hour was used as a baseline or template to compare. Data analysis implied that the neural assemblies during SWS correlated significantly more with templates than during waking hours or REM sleep. As well, these post-learning, post-SWS reverberations lasted 48 hours, much longer than the duration of novel object learning (one hour), indicating long term potentiation. In order to monitor this activation, experimenters used PET scans and fMRI to use cerebral blood flow as a marker of synaptic activity. The finding noted that the amount of hippocampal activation during slow-wave sleep positively correlated to the improvement on the virtual tour task the following day, which indicates that hippocampal activity during sleep correlates with the improvement in memory performance. These findings prove that learning-dependent modulation in hippocampal activity while sleeping shows processing of the previously learned episodic and spatial memory traces. This modulation of the hippocampus leads to plastic changes in the brain and ultimately an improvement in performance. The results of this study showed that spatial memory traces are processed in humans while they are in NREM sleep. It showed a reaction of the hippocampal formation during SWS, after a declarative spatial memory task. Experimenters also found, that in humans, there is experience-dependent modulation of activity during NREM sleep in the hippocampal regions, but not during REM sleep after learning. The evidence from this study was substantial to its hypothesis that the information learned while awake is altered, and strengthened while humans are sleeping. It assumes the hippocampus might hold information only temporarily and in fast-learning rate, whereas the neocortex is related to long-term storage and slow-learning rate. This dialogue between hippocampus and neocortex occurs in parallel with hippocampal sharp-wave ripples and thalamo-cortical spindles, synchrony that drives the formation of spindle-ripple event which seems to be a prerequisite for the formation of long-term memories. A study using 49 rats indicated the increase of sleep spindles during slow-wave sleep following learning. It gave evidence to the increase of spindle frequency during non-REM sleep following paired associate of motor-skill learning tasks. Using an EEG, sleep spindles were detected and shown to be present only during slow-wave sleep. Beginning with a preliminary study, rats underwent six hours of monitored sleep, after a period of learning. Results showed that during the first hour following learning, there was the most evident effect on learning-modulated sleep spindle density. However, this increase in spindle density was not dependent on the training condition. In other words, there was an increase in spindles regardless of how the rats were trained. EEG patterns showed a significant difference in the density of sleep spindles compared to the density of a control group of rats, who did not undergo any training before their sleep spindles were measured. This effect of increased spindle density only lasted for the first hour into sleep following training, and then disappeared within the second hour into sleep. Reward learning and memory In a study by Fischer and Born, 2009, previous knowledge of monetary reward and post-training sleep are proven to be significant predictors of overall finger sequence tapping performance. Subjects were presented with two different finger sequence tasks that would have to be replicated at a later time. The subjects were told that there would be a reward offered for improvement upon a specific finger tapping sequence task. A control group was not given any knowledge of a reward. The subjects were separated further by allowing a sleep period between initial training and final testing for one group while another group faced a wake retention interval. It was concluded that the group that received both information about reward as well as being able to sleep displayed the highest increase in performance on both finger tapping sequences. Knowledge of reward without sleep and sleep without knowledge of reward were both significant contributors to improved performance. In all cases sleep was determined to have an advantageous effect on overall performance when compared to groups that underwent a twelve-hour wake retention period. == Non-declarative memory ==

Non-declarative memory is memory gained from previous experiences that is unconsciously applied to everyday scenarios. Non-declarative memory is essential for the performance of learned skills and habits, for example, running or cooking a favourite meal. There are three types of non-declarative memories: implicit memory (unconscious memory, priming), instrumental memory (classical conditioning), and procedural memory (automatic skill memory). Sleep deprivation ERK phosphorylation Extracellular signal-related kinases, also known as classical MAP kinase, are a group of protein kinases located in neurons. These proteins are activated or deactivated by phosphorylation (adding of a phosphate group using ATP), in response to neurotransmitters and growth factors. This can result in subsequent protein to protein interactions and signal transductions (neurotransmitters or hormones transmit to cells), which ultimately controls all cellular processes including gene transcription and cell cycles (important in learning and memory). A study tested four groups of rats in the Morris Water Maze, two groups in the spatial task (hidden platform) and two groups in the non-spatial task (visible platform.) The effects of six hours of total sleep deprivation (TSD) were assessed for the experimental group (one spatial group, one non-spatial group) in both tasks. Six hours after the TSD period (or sleep period for controls), the groups of rats were trained on either task then tested 24 hours later. In addition, the levels of total ERK phosphorylation (ERK 1 and ERK 2), protein phosphate 1 (PP1), and MAPK phosphatase 2 (latter two both involved in dephosphorylation) were assessed by decapitating four other groups of mice, (two sleep deprived and two non-sleep deprived), and removing their hippocampuses after the six hours of TSD, or two hours after TSD (eight hours total). Results showed that TSD did not impair learning of the spatial task, but it did impair memory. With regards to the non-spatial task, learning again was no different in the TSD; however, memory in the TSD group was actually slightly better, although not quite significantly. Analysis of the hippocampus showed that TSD significantly decreased the levels of total ERK phosphorylation by about 30%. TSD did not affect proteins in the cortex which indicates that the decreases in ERK levels were due to impaired signal transduction in the hippocampus. In addition, neither PP1 or MAPK phosphatase 2 levels were increased suggesting that the decreases in ERK were not due to dephosphorylation but instead a result of TSD. Therefore, it is proposed that TSD has aversive effects on the cellular processes (ERK: gene transcription etc.), underlying sleep-dependent memory plasticity. used an avoidance task followed by a post-training REM sleep period to examine changes in P waves affecting reprocessing of recently acquired stimuli. It was found that not only were the P waves increased during post-training sleep but also the density of the waves. These findings may imply that P waves during REM sleep may help to activate critical forebrain and cortical structures dealing with memory consolidation. In a Hennevin et al. study, 1989, the mesencephalic reticular formation (MRF) was given light electrical stimulation, during REM sleep, which is known to have an advantageous effect for learning when applied after training. The rats in the experiment were trained to run a maze in search of a food reward. One group of rats was given non-awakening MRF electrical stimulations after each of their maze trials compared to a control group which did not receive any electrical stimulation. It was noticed that the stimulated rats performed significantly better in respect to error reduction. These findings imply that dynamic memory processes occur both during training as well as during post-training sleep. Another study by Hennevin et al. (1998) conditioned rats to fear a noise that is associated with a subsequent foot shock. The interesting part of the experiment is that fear responding to the noise (measured in the amygdala) was observed when the noise was presented during REM sleep. This was compared to a group of pseudo-conditioned rats who did not display the same amygdalar activation during post-training sleep. This would suggest that neural responding to previously salient stimuli is maintained even during REM sleep. There is no shortage of research conducted on the effects that REM sleep has on the working brain, but consistency in the findings is what plagues recent research. There is no guarantee as to what functions REM sleep may perform for our bodies and brains, but modern research is always expanding and assimilating new ideas to further our understanding of such processes. PGO waves In animals, the appearance of ponto-geniculo-occipital waves (PGO waves) is related to that of the bioelectric outputs of rapid eye movements. These waves are most clearly seen during the transition from non-REM to REM sleep. Although these phasic waves are observed in many portions of the animal brain, they are most noticeable in the pons, lateral geniculate bodies, and the occipital cortex. Peigneux et al., 2006, In a study using post learning REM sleep deprivation the effects of stimulating the P wave generator (located in the pontine tegmentum) of a rat were observed. Two groups of rats underwent an avoidance learning task and then allowed a sleep period while another group of rats were deprived sleep. When comparing the two groups the sleep deprived rats showed a significant deficit in learning from having not undergone REM sleep. In another rat group, the P wave generator was stimulated using a carbachol injection and the rats then underwent a sleep deprivation stage. When these rats were again tested on their learning it was shown that activation of the P wave generator during sleep deprivation resulted in normal learning being achieved. This would point to the fact that the activation of P waves, even without REM sleep, was enough to enhance the memory processes that would not normally have happened. Implicit face memory Faces are an important part of one's social life. To be able to recognize, respond and act towards a person requires unconscious memory encoding and retrieval processes. Facial stimuli are processed in the fusiform gyrus (occipito-temporal brain area) and this processing is an implicit function representing a typical form of implicit memory. REM sleep has been seen to be more beneficial to implicit visuospatial memory processes, rather than slow-wave sleep which is crucial for explicit memory consolidation. REM sleep is known for its visual experiences, which may often include detailed depictions of the human countenance. However, control subjects did not complete a SRT task, thus researchers could not assume the reactivation of certain networks to be a result of the implicitly learned sequence/grammar as it could simply be due to elementary visuomotor processing which was obtained in both groups. To answer this question the experiment was redone and another group was added who also took part in the SRT task. They experienced no sequence to the SRT task (random group), whereas the experimental group did experience a sequence (probabilistic group), although without conscious awareness. Results of PET scans indicate that bilateral cuneus were significantly more activated during SRT practice as well as post-training REM sleep in the Probabilistic group than the Random group. Sei et al., inserted electrodes into the skulls of seven pairs of rats to measure electroencephalogram (EEG), and inserted wires into the neck muscles of the rats to measure electromyogram (EMG), a technique used to measure the amount of muscle activity. Half the rats experienced a six-hour REM sleep deprivation period, while the other half experienced a six-hour sleep period, containing all sleep cycles. Results showed that the rats in the REM sleep deprivation group experienced decreased levels of brain-derived neurotrophic factor in the cerebellum (coordination, motor learning) and brainstem (sensory and motor ascending pathway); conversely, the hippocampus (long-term memory, spatial navigation), showed decreases in nerve growth factor levels. BDNF protein has been shown to be necessary for procedural learning (form of non-declarative memory). Since procedural learning has also exhibited consolidation and enhancement under REM sleep, it is proposed that the impairment of procedural learning tasks is due to the lack of BDNF proteins in the cerebellum and brainstem during RSD. conducted a finger sequence tapping experiment in which the subjects were shown coloured dots in sequence on a monitor corresponding to buttons on their keyboard. When a colour was shown, the subject had to react by pressing the right colour on the keyboard. The subjects were separated into three groups. Group one continually trained with no periods of sleep. Group two was trained and retested over ten hours of wakefulness followed by eight hours of sleep and final testing. The third group was trained at ten pm, followed by an eight-hour sleep. This group was then tested the following morning and again later in the same day. Results showed that wakefulness was an insignificant predictor of performance improvement, unless followed by a period of sleep. Groups that were allowed a post training sleep period, regardless of its time in reference to training, experienced improvements in learning the finger tapping sequences. The initial working memory capacity of the groups averaged three to four units. In groups two and three, the working memory capacity was increased to an average of 5–6 units. It was proposed that sleep-dependent improvements may contribute to overall improvement in working memory capacity, leading to improved fluid intelligence. Sleep deprivation Sleep deprivation, whether it is total sleep deprivation or partial sleep deprivation, can impair working memory in measures of memory, speed of cognitive processing, attention and task switching. Casement et al. found that when subjects were asked to recognize digits displayed on a screen by typing them on a keypad, the working memory speed of subjects whose sleep was restricted to four hours a night (approximately 50% of their normal sleep amount) were 58% slower than control groups who were allowed their full eight hours of sleep. ==Synaptic plasticity==

The brain is an ever-changing, plastic, model of information sharing and processing. In order for the brain to incorporate new experiences into a refined schema, it has to undergo specific modifications to consolidate and assimilate all new information. Using a pursuit task (used to test visuomotor capabilities) in combination with an fMRI, Maquet et al., 2003, found that increases in activation were seen in the supplementary eye field and right dentate nucleus of subjects who were allowed to sleep as compared to sleep deprived individuals. The right superior temporal sulcus was also noticed to have higher activation levels. When functional connectivity was analyzed it was found that the dentate nucleus was more closely involved with the functions of the superior temporal sulcus. The results suggest that performance on the pursuit task relies on the subject's ability to comprehend appropriate movement patterns in order to recreate the optimal movements. Sleep deprivation was found to interrupt the slow processes that lead to learning of this procedural skill and alter connectivity changes that would have normally been seen after a night of rest. Neuroplasticity has been thoroughly researched over the past few decades and results have shown that significant changes that occur in our cortical processing areas have the power to modulate neuronal firing to both new and previously experienced stimuli. Neurotransmitter regulation The changes in quantity of a certain neurotransmitter as well as how the post-synaptic terminal responds to this change are underlying mechanisms of brain plasticity. Evidence has shown that functioning of the hippocampus-dependent memory system (episodic memory and autobiographical memory) is directly affected by cholinergic changes throughout the wake-sleep cycle. High levels of ACh would promote information attained during wakefulness to be stored in the hippocampus. This is accomplished by suppressing previous excitatory connections while facilitating encoding without interference from previously stored information. During NREM sleep, and especially slow-wave sleep, low levels of Ach would cause the release of this suppression and allow for spontaneous recovery of hippocampal neurons resulting in the facilitation of memory consolidation. Gene expression Recently, approximately one hundred genes whose brain expression is increased during periods of sleep have been found. A similar number of genes were found to promote gene expression during wakefulness. These sets of genes are related to different functional groups which may promote different cellular processes. The genes expressed during wakefulness may perform numerous duties including energy allocation, synaptic excitatory neurotransmission, high transcriptional activity and synaptic potentiation in learning of new information. There was a sleep related increase in processes that involve the synthesis and maintenance of the synapse. Such processes include membrane trafficking, synaptic vesicle recycling, myelin structural protein formation, and cholesterol and protein synthesis. In a different study it was found that there was a sleep related increase in calmodulin-dependent protein kinase IV that has been specifically involved in synaptic depression and in the consolidation of long-term memory. These findings encourage an association between sleep and different aspects of neural plasticity. Recordings from previous experiments demonstrate that sleep reorganizes and restructures neural firing sequences and synaptic potency in the hippocampus. When asleep, hippocampal neurons shift their firing patterns and adjust synaptic strengths in ways that support the consolidation and synthesis of new information. == Alternative sleep schedules ==

Motor skills learning The impact of daytime naps was looked at by Walker and Stickgold (2005). Visuals skills learning Much like motor skills learning, visual skills learning increased after a daytime nap period. Researchers Mednick and colleagues have shown that if a visual skills task (find task) is taught in the morning and repeatedly tested throughout the day, individuals will actually become worse at the task. The individuals that were allowed a 30-60 minute nap seemed to gain stabilization of the skill, as no deterioration occurred. If allowed a 60-90 minute nap (REM sleep and slow-wave sleep), individuals displayed enhancement. Unlike the motor task, enhancement was not suppressed during the nocturnal sleep if the individual had napped earlier. In the situation of visual skill learning, naps have been shown to prevent wakeful deterioration and even enhance learning above and beyond enhancement occurring in nocturnal sleep. Shift workers Shift workers who work throughout the night have been known to have far more accidents as opposed to daytime workers. This can be attributed to several factors, including fewer staff and fatigue; however, part of the problem may be the workers' poor working memory and poor performance skills due to poor memory consolidation. Both implicitly learned tasks and explicitly learned tasks improve by roughly 20% after a full nights sleep. Without an adequate nights sleep between learning a new task and performance of that task, performance fails to improve. Shift workers who are not given an adequate amount of sleep, particularly in the NREM stage, between learning and performance of a task will not perform as well as workers who maintain a standard sleep routine. ==Sleep and aging==

Sleep often becomes deregulated in the elderly, a problem which can lead to or exacerbate pre-existing memory decline. Sleep quality between ages 50 to 70 significantly predicts cognitive functions. Healthy older adults The positive correlation between sleep and memory breaks down with aging. In general, older adults suffer from decreased sleep efficiency. The amount of time and density of REM sleep and slow wave sleep (SWS) decreases with age. Consequently, it is common that the elderly receive no increase in memory after a period of rest. With age, sleep appears to lose its cognitive benefits. Younger adults, around 19- and 20-years old show a strong positive correlation between SWS and memory retention, but older adults around 70- and 71-years old show no positive correlation between SWS and memory. Additionally, older adults demonstrated a negative correlation between SWS and new learning. Older adults show less activation in the hippocampal during SWS, weakening memory consolidation. To combat this, donepezil has been tested in healthy elderly patients where it was shown to increase time spent in REM sleep and improve following day memory recall. Donepezil was shown to enhance cholinergic activity, increasing REM density (more eye movement and faster entry to REM sleep) and supporting overnight memory consolidation. By age 50, sleep quality begins to impact brain functioning. Poor sleep quality has been linked to decrease cognitive functions such as executive abilities; the strongest impairment is seen around age 62. Crystallized cognition and skills related to language and accumulated knowledge show sensitivity to sleep quality between ages 66 and 70, suggesting a sensitive age window between ages 50 to 70. Alzheimer's disease Alzheimers disease is thought to be caused by the abnormal buildup of proteins around brain cells which disrupt the activity of neurotransmitters. Patients with Alzheimer's disease experience more sleep disruption than the healthy elderly. Studies have shown that in patients with Alzheimer's disease, there is a decrease in fast spindles. It has also been reported that spindle density the night before a memory test correlates positively with accuracy on an immediate recall task. ==See also==