Relationship between sleep and memory
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.
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:
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.
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).
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