Sleep and memory

The relationship between sleep and memory has been postulated and studied since at least the early 19th century. Memory, the cognitive process whereby experiences, learning and recognition are recalled, 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 (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. The relationship between sleep and memory has been postulated and studied since at least the early 19th century. Memory, the cognitive process whereby experiences, learning and recognition are recalled, 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 (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 even 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. In 1801, David Hartley first postulated that dreaming altered the associative planetary links within the brain during dreaming periods of dreams. 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 mental 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 progresses in a cyclical fashion through 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 EEG measures to be characterized by beta waves which are the highest in frequency and 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. 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. 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 you have 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.