Multiple hypotheses explain the possible connections between sleep and learning in humans. Research indicates that sleep does more than allow the brain to rest; it may also aid the consolidation of long-term memories.
REM sleep and slow-wave sleep play different roles in memory consolidation. REM is associated with the consolidation of nondeclarative (implicit) memories. An example of a nondeclarative memory would be a task that we can do without consciously thinking about it, such as riding a bike. Slow-wave, or non-REM (NREM) sleep, is associated with the consolidation of declarative (explicit) memories. These are facts that need to be consciously remembered, such as dates for a history class. [1]
Popular sayings can reflect the notion that remolded memories produce new creative associations in the morning, and that performance often improves after a time-interval that includes sleep. [2] Current studies demonstrate that a healthy sleep produces a significant learning-dependent performance boost. [3] [4] The idea is that sleep helps the brain to edit its memory, looking for important patterns and extracting overarching rules which could be described as 'the gist', and integrating this with existing memory. [5] The 'synaptic scaling' hypothesis suggests that sleep plays an important role in regulating learning that has taken place while awake, enabling more efficient and effective storage in the brain, making better use of space and energy. [6]
Healthy sleep must include the appropriate sequence and proportion of NREM and REM phases, which play different roles in the memory consolidation-optimization process. During a normal night of sleep, a person will alternate between periods of NREM and REM sleep. Each cycle is approximately 90 minutes long, containing a 20-30 minute bout of REM sleep. [7] NREM sleep consists of sleep stages 1–4, and is where movement can be observed. A person can still move their body when they are in NREM sleep. If someone sleeping turns, tosses, or rolls over, this indicates that they are in NREM sleep. REM sleep is characterized by the lack of muscle activity. Physiological studies have shown that aside from the occasional twitch, a person actually becomes paralyzed during REM sleep. [7] In motor skill learning, an interval of sleep may be critical for the expression of performance gains; without sleep these gains will be delayed. [8]
Procedural memories are a form of nondeclarative memory, so they would most benefit from the fast-wave REM sleep. [7] In a study, [9] procedural memories have been shown to benefit from sleep. [10] Subjects were tested using a tapping task, where they used their fingers to tap a specific sequence of numbers on a keyboard, and their performances were measured by accuracy and speed. This finger-tapping task was used to simulate learning a motor skill. The first group was tested, retested 12 hours later while awake, and finally tested another 12 hours later with sleep in between. The other group was tested, retested 12 hours later with sleep in between, and then retested 12 hours later while awake. The results showed that in both groups, there was only a slight improvement after a 12-hour wake session, but a significant increase in performance after each group slept. This study gives evidence that REM sleep is a significant factor in consolidating motor skill procedural memories, therefore sleep deprivation can impair performance on a motor learning task. This memory decrement results specifically from the loss of stage 2, REM sleep. [11]
Declarative memory has also been shown to benefit from sleep, but not in the same way as procedural memory. Declarative memories benefit from the slow-waves nREM sleep. [7] A study [12] was conducted where the subjects learned word pairs, and the results showed that sleep not only prevents the decay of memory, but also actively fixates declarative memories. [13] Two of the groups learned word pairs, then either slept or stayed awake, and were tested again. The other two groups did the same thing, except they also learned interference pairs right before being retested to try to disrupt the previously learned word pairs. The results showed that sleep was of some help in retaining the word pair associations, while against the interference pair, sleep helped significantly.
After sleep, there is increased insight. This is because sleep helps people to reanalyze their memories. The same patterns of brain activity that occur during learning have been found to occur again during sleep, only faster. One way that sleep strengthens memories is by weeding out the less successful connections between neurons in the brain. This weeding out is essential to prevent overactivity. The brain compensates for strengthening some synapses (connections) between neurons, by weakening others. The weakening process occurs mostly during sleep. This weakening during sleep allows for strengthening of other connections while we are awake. Learning is the process of strengthening connections, therefore this process could be a major explanation for the benefits that sleep has on memory. [14]
Research has shown that taking an afternoon nap increases learning capacity. A study [15] tested two groups of subjects on a nondeclarative memory task. One group engaged in REM sleep, and one group did not (meaning that they engaged in NREM sleep). The investigators found that the subjects who engaged only in NREM sleep did not show much improvement. The subjects who engaged in REM sleep performed significantly better, indicating that REM sleep facilitated the consolidation of nondeclarative memories. [7] A more recent study [16] demonstrated that a procedural task was learned and retained better if it was encountered immediately before going to sleep, while a declarative task was learned better in the afternoon. [6]
A 2009 study [17] based on electrophysiological recordings of large ensembles of isolated cells in the prefrontal cortex of rats revealed that cell assemblies that formed upon learning were more preferentially active during subsequent sleep episodes. More specifically, those replay events were more prominent during slow wave sleep and were concomitant with hippocampal reactivation events. This study has shown that neuronal patterns in large brain networks are tagged during learning so that they are replayed, and supposedly consolidated, during subsequent sleep. There have been other studies that have shown similar reactivation of learning pattern during motor skill and neuroprosthetic learning. [18] [19] Notably, new evidence is showing that reactivation and rescaling may be co-occurring during sleep. [20]
Sleep has been directly linked to the grades of students. One in four U.S. high school students admit to falling asleep in class at least once a week. [21] Consequently, results have shown that those who sleep less do poorly. In the United States, sleep deprivation is common with students because almost all schools begin early in the morning and many of these students either choose to stay awake late into the night or cannot do otherwise due to delayed sleep phase syndrome. [22] As a result, students that should be getting between 8.5 and 9.25 hours of sleep are getting only 7 hours. [23] Perhaps because of this sleep deprivation, their grades are lower and their concentration is impaired. [24]
As a result of studies showing the effects of sleep deprivation on grades, and the different sleep patterns for teenagers, a school in New Zealand changed its start time to 10:30 a.m. in 2006, to allow students to keep to a schedule that allowed more sleep. In 2009, Monkseaton High School, in North Tyneside, had 800 pupils aged 13–19 starting lessons at 10 a.m. instead of the normal 9 a.m. and reported that general absence dropped by 8% and persistent absenteeism by 27%. [25] Similarly, a high school in Copenhagen [ which? ] has committed to providing at least one class per year for students which will start at 10 a.m. or later.
College students represent one of the most sleep-deprived segments of the population. Only 11% of American college students sleep well, and 40% of students feel well rested only two days per week. About 73% have experienced at least some occasional sleep issues. This poor sleep is thought to have a severe impact on their ability to learn and remember information because the brain is being deprived of time that it needs to consolidate information which is essential to the learning process. [26]
Long-term memory (LTM) is the stage of the Atkinson–Shiffrin memory model in which informative knowledge is held indefinitely. It is defined in contrast to short-term and working memory, which persist for only about 18 to 30 seconds. LTM is commonly labelled as "explicit memory" (declarative), as well as "episodic memory," "semantic memory," "autobiographical memory," and "implicit memory".
Sleep is a state of reduced mental and physical activity in which consciousness is altered and sensory activity is inhibited to a certain extent. During sleep, there is a decrease in muscle activity, and interactions with the surrounding environment. While sleep differs from wakefulness in terms of the ability to react to stimuli, it still involves active brain patterns, making it more reactive than a coma or disorders of consciousness.
Rapid eye movement sleep is a unique phase of sleep in humans, mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vividly.
Muscle memory is a form of procedural memory that involves consolidating a specific motor task into memory through repetition, which has been used synonymously with motor learning. When a movement is repeated over time, the brain creates a long-term muscle memory for that task, eventually allowing it to be performed with little to no conscious effort. This process decreases the need for attention and creates maximum efficiency within the motor and memory systems. Muscle memory is found in many everyday activities that become automatic and improve with practice, such as riding bikes, driving motor vehicles, playing ball sports, typing on keyboards, entering PINs, playing musical instruments, poker, martial arts, swimming, and dancing.
Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during these stages. Dreaming occurs during both sleep states, and muscles are not paralyzed as in REM sleep. People who do not go through the sleeping stages properly get stuck in NREM sleep, and because muscles are not paralyzed a person may be able to sleepwalk. According to studies, the mental activity that takes place during NREM sleep is believed to be thought-like, whereas REM sleep includes hallucinatory and bizarre content. NREM sleep is characteristic of dreamer-initiated friendliness, compared to REM sleep where it's more aggressive, implying that NREM is in charge of simulating friendly interactions. The mental activity that occurs in NREM and REM sleep is a result of two different mind generators, which also explains the difference in mental activity. In addition, there is a parasympathetic dominance during NREM. The reported differences between the REM and NREM activity are believed to arise from differences in the memory stages that occur during the two types of sleep.
Explicit memory is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and concepts. This type of memory is dependent upon three processes: acquisition, consolidation, and retrieval.
Sleep spindles are bursts of neural oscillatory activity that are generated by interplay of the thalamic reticular nucleus (TRN) and other thalamic nuclei during stage 2 NREM sleep in a frequency range of ~11 to 16 Hz with a duration of 0.5 seconds or greater. After generation as an interaction of the TRN neurons and thalamocortical cells, spindles are sustained and relayed to the cortex by thalamo-thalamic and thalamo-cortical feedback loops regulated by both GABAergic and NMDA-receptor mediated glutamatergic neurotransmission. Sleep spindles have been reported for all tested mammalian species. Considering animals in which sleep-spindles were studied extensively, they appear to have a conserved main frequency of roughly 9–16 Hz. Only in humans, rats and dogs is a difference in the intrinsic frequency of frontal and posterior spindles confirmed, however.
Slow-wave sleep (SWS), often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep. It usually lasts between 70 and 90 minutes and takes place during the first hours of the night. Initially, SWS consisted of both Stage 3, which has 20–50 percent delta wave activity, and Stage 4, which has more than 50 percent delta wave activity.
Recurrent thalamo-cortical resonance is an observed phenomenon of oscillatory neural activity between the thalamus and various cortical regions of the brain. It is proposed by Rodolfo Llinas and others as a theory for the integration of sensory information into the whole of perception in the brain. Thalamocortical oscillation is proposed to be a mechanism of synchronization between different cortical regions of the brain, a process known as temporal binding. This is possible through the existence of thalamocortical networks, groupings of thalamic and cortical cells that exhibit oscillatory properties.
Ponto-geniculo-occipital waves or PGO waves are distinctive wave forms of propagating activity between three key brain regions: the pons, lateral geniculate nucleus, and occipital lobe; specifically, they are phasic field potentials. These waves can be recorded from any of these three structures during and immediately before REM sleep. The waves begin as electrical pulses from the pons, then move to the lateral geniculate nucleus residing in the thalamus, and end in the primary visual cortex of the occipital lobe. The appearances of these waves are most prominent in the period right before REM sleep, albeit they have been recorded during wakefulness as well. They are theorized to be intricately involved with eye movement of both wake and sleep cycles in many different animals.
Memory consolidation is a category of processes that stabilize a memory trace after its initial acquisition. A memory trace is a change in the nervous system caused by memorizing something. Consolidation is distinguished into two specific processes. The first, synaptic consolidation, which is thought to correspond to late-phase long-term potentiation, occurs on a small scale in the synaptic connections and neural circuits within the first few hours after learning. The second process is systems consolidation, occurring on a much larger scale in the brain, rendering hippocampus-dependent memories independent of the hippocampus over a period of weeks to years. Recently, a third process has become the focus of research, reconsolidation, in which previously consolidated memories can be made labile again through reactivation of the memory trace.
Procedural memory is a type of implicit memory which aids the performance of particular types of tasks without conscious awareness of these previous experiences.
It has been estimated that over 20% of adults suffer from some form of sleep deprivation. Insomnia and sleep deprivation are common symptoms of depression, and can be an indication of other mental disorders. The consequences of not getting enough sleep could have dire results; not only to the health, cognition, energy level and the mood of the individual, but also to those around them as sleep deprivation increases the risk of human-error related accidents, especially with vigilance-based tasks involving technology.
The relationship between sleep and memory has been studied since at least the early 19th century. Memory, the cognitive process of storing and retrieving past experiences, learning and recognition, is a product of brain plasticity, the structural changes within synapses that create associations between stimuli. Stimuli are encoded within milliseconds; however, the long-term maintenance of memories can take additional minutes, days, or even years to fully consolidate and become a stable memory that is accessible. Therefore, the formation of a specific memory occurs rapidly, but the evolution of a memory is often an ongoing process.
The activation-synthesis hypothesis, proposed by Harvard University psychiatrists John Allan Hobson and Robert McCarley, is a neurobiological theory of dreams first published in the American Journal of Psychiatry in December 1977. The differences in neuronal activity of the brainstem during waking and REM sleep were observed, and the hypothesis proposes that dreams result from brain activation during REM sleep. Since then, the hypothesis has undergone an evolution as technology and experimental equipment has become more precise. Currently, a three-dimensional model called AIM Model, described below, is used to determine the different states of the brain over the course of the day and night. The AIM Model introduces a new hypothesis that primary consciousness is an important building block on which secondary consciousness is constructed.
The neuroscience of sleep is the study of the neuroscientific and physiological basis of the nature of sleep and its functions. Traditionally, sleep has been studied as part of psychology and medicine. The study of sleep from a neuroscience perspective grew to prominence with advances in technology and the proliferation of neuroscience research from the second half of the twentieth century.
Sharp waves and ripples (SWRs) are oscillatory patterns produced by extremely synchronised activity of neurons in the mammalian hippocampus and neighbouring regions which occur spontaneously in idle waking states or during NREM sleep. They can be observed with a variety of imaging methods, such as EEG. They are composed of large amplitude sharp waves in local field potential and produced by tens of thousands of neurons firing together within 30–100 ms window. They are some of the most synchronous oscillations patterns in the brain, making them susceptible to pathological patterns such as epilepsy.They have been extensively characterised and described by György Buzsáki and have been shown to be involved in memory consolidation in NREM sleep and the replay of memories acquired during wakefulness.
Hippocampal replay is a phenomenon observed in rats, mice, cats, rabbits, songbirds and monkeys. During sleep or awake rest, replay refers to the re-occurrence of a sequence of cell activations that also occurred during activity, but the replay has a much faster time scale. It may be in the same order, or in reverse. Cases were also found where a sequence of activations occurs before the actual activity, but it is still the same sequence. This is called preplay.
Jan Born is a neuroscientist who researches the role of sleep in memory consolidation, problem solving, and brain plasticity. He is Head of the Institute of Medical Psychology and the Behavioral Neurobiology department at the University of Tübingen.
Gina R. Poe is an American neuroscientist specializing in the study of sleep and its effect on memory and learning. Her findings have shown that the absence of noradrenaline and low levels of serotonin during sleep spindles allow the brain to form new memories during REM, as well as restructure old memory circuits to allow for more learning during later waking periods. She currently works as a professor at the University of California, Los Angeles (UCLA).
