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 (more resistant to change or interference). Therefore, the formation of a specific memory occurs rapidly, but the evolution of a memory is often an ongoing process.
Memory processes have been shown to be stabilized and enhanced (sped up and/or integrated) and memories better consolidated by nocturnal sleep and daytime naps. Certain sleep stages have been demonstrated as improving an individual's memory, though this is task-specific. Generally, declarative memories are believed to be enhanced by slow-wave sleep, while non-declarative memories are enhanced by rapid eye movement (REM) sleep, although there are some inconsistencies among experimental results. The effect of sleep on memory, especially as it pertains to the human brain, is an active field of research in neurology, psychology, and related disciplines.
History 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. Their results showed that memory retention was much better after a period of sleep compared to the same time interval spent awake. It was not until 1953, however, when sleep was delineated into rapid eye movement sleep and non-rapid eye movement sleep, that studies focusing on the effect of specific sleep stages on memory were conducted. 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 processes vs. experience-dependent processes Brain activity that occurs during sleep is assessed in two ways: Use-dependency, and Experience-dependency. 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.
Experience-dependent brain activity is a result of a new situation, environment, or learned task or fact that has taken place in the pre-sleep period. This is the type of brain activity that denotes memory consolidation/enhancement.
It is often hard to distinguish between the two in an experimental setting because the setting alone is a new environment. This new environment would be seen in the sleeping brain activity along with the newly learned task. To avoid this, most experimenters insist participants spend one day in the experimental condition before testing begins so the setting is not novel once the experiment begins. This ensures the collected data for experience-dependent brain activity is purely from the novel task.
Consolidation
Consolidation of a memory is a process that takes an initially unstable representation and encodes it in a more sturdy, effective and efficient manner. In this new state, the memory is less susceptible to interference. There are essentially three phases of memory consolidation and all are thought to be facilitated by sleep or not sleep: Stabilization is the encoding of a memory which takes only 6 milliseconds. Enhancement is the continual process of consolidation which can occur over minutes, 7 hours, days but not longer. Post-sleep behavioural activities can be seen to show significant improvements in the absence of practice. Integration can also take hours or years and is the process of connecting recently encoded memories into existing memory networks.
Reconsolidation Reconsolidation of a memory involves the retrieval of an already consolidated memory (explicit or implicit), into short-term or working memory. Here it is brought into a labile state where subsequent information can interfere with what is currently in memory, therefore altering the memory. This is known as retroactive interference, and is an extremely significant issue for court and eyewitness testimonies.
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 route retrieval task spatial learning occurs where a participant virtual tours a particular place (such as a town or maze). Participants are asked to virtually tour the same thing at a later time while brain imaging is used to measure activity. A paired word associative task consists of two phases. During the first phase (acquisition), the responses of the paired-associate task are learned and become recallable. In the second phase (associative phase), the subject learns to pair each response to a separate stimulus. For example, a visual cue would provide information as to what words must be recalled after the stimulus and words are removed. 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 Morris water maze task rats are used to test their spatial learning in two kinds of conditions: spatial and nonspatial. In the spatial condition, a platform is hidden by using murky water and in the nonspatial condition, the platform is visible. The spatial condition the rat must rely on their spatial memory to find the platform whereas the nonspatial condition is used for comparison purposes.
The serial reaction time task (SRT task) is a task whereby subjects face a computer screen where several markers are displayed that are spatially related to relevant markers on their keyboard. The subjects are asked to react as fast and accurately as possible to the appearance of a stimulus below one of the markers. Subjects can be trained on the task with either explicit instructions (e.g. there are colour sequences presented which must be learned) or implicit ones (e.g. the experimenter does not mention colour sequences, thus leaving the subjects to believe that they are taking place in a speed test). When this task is used in sleep studies, after a time delay, subjects are tested for retention. 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.