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.
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