The role of slow wave sleep in memory processing.

T he functions of sleep have been extensively studied and debated, but remain largely unknown. However, evidence has demonstrated the necessity of sleep; sleep is tightly regulated , it has a restorative nature and, if eliminated completely, can ultimately lead to death. 1 Perhaps the earliest reference to the beneficial impact of sleep on memory was by the Roman rhetorician, Quintilian, who stated: " …[it] is a curious fact, of which the reason is not obvious , that the interval of a single night will greatly increase the strength of the memory… Whatever the cause, things which could not be recalled on the spot are easily coordinated the next day, and time itself, which is generally accounted one of the causes of forgetfulness, actually serves to strengthen the memory. " 2 In the early 18th century, it was proposed that dreaming might be associated with memory links within the brain, 3 while in the 1920s, it was demonstrated experimentally that sleep, rather than periods of time awake, may be more closely associated with the strength of a memory. 4 However, it was not until the 1950s, with the discovery of rapid eye movement (REM) and nonREM (NREM) sleep, 5 that research progressed to evaluate the relationship between the different stages of sleep and the process of memory development. Evidence now suggests that sleep is important in the processing of newly acquired information and for the long-term storage of memory. This has become known as " sleep-dependent memory processing. " Memories can be initially formed or " encoded " when the brain engages in an idea, image, thought, experience , or action, leading to the formation of a representation of this information in the brain. However, following encoding, this memory then appears to require " consolidation, " which refers to the process of memory stabilization over time, making it more resistant to interference or disruption. Memories can also be reconsolidated should they become destabilized, deteriorate, or require enhancement. 6 Sleep has been implicated in all of these processes. Indeed, impaired memory consolidation has been demonstrated in patients with primary insomnia; such patients do not perform as well as healthy control subjects in tests of memory following a period of sleep. 7,8 More specifically, the ability to form and retain nonemotional, fact-based (" episodic ") memories has been linked to the presence and integrity of slow wave sleep (SWS). Compared with healthy …