Res Medica, Volume 21, Issue 1 Page 1 of 13 REVIEW ARTICLE A sleep to remember: The effects of sleep on memory Deep P Sarode1, Iain D Mathie1, Neng P Gao1, Lewis I Gray1, Ian J Monaghan1, Andrew P Preston1, Matthew J Twomey1, Marianne Watters1 1 Year 3 MBChB, University of Edinburgh Correspondence email: [email protected] Abstract For centuries, the functions of sleep have been researched. Multiple theories have been developed, but even now, scientists are unable to produce a conclusive explanation as to why we sleep. It is evident that sleep is vital, as even in animals, it has been argued that sleep deprivation leads to serious consequences. More recently, research has suggested that sleep plays a role in memory consolidation. This review aims to bring together the evidence concerning the link between sleep and different memory sub-classifications (episodic memory, semantic memory, procedural memory and conditioning) and its potential clinical application will be discussed. Copyright Royal Medical Society. All rights reserved. The copyright is retained by the author and the Royal Medical Society, except where explicitly otherwise stated. Scans have been produced by the Digital Imaging Unit at Edinburgh University Library. Res Medica is supported by the University of Edinburgh’s Journal Hosting Service: http://journals.ed.ac.uk ISSN: 2051-7580 (Online) ISSN: 0482-3206 (Print) Res Medica is published by the Royal Medical Society, 5/5 Bristo Square, Edinburgh, EH8 9AL Res Medica, 2013, 21(1):23-34 doi: 10.2218/resmedica.v21i1.179 Sarode DP, et al. A Sleep to Remember: The Effects of Sleep on Memory, Res Medica 2013, 21(1), pp.23-34 doi:10.2218/resmedica.v21i1.179 Sarode DP et al. http://journals.ed.ac.uk/resmedica Introduction regarding the latter two. Procedural memory An increasing area of interest in the field involves learning a motor skill, such as of neuroscience is the potential link between learning to swim. Conditioning memory is REVIEW sleep and memory. Recent years have seen the learnt response to a once neutral growing evidence supporting the idea that stimulus. The classical example is Pavlov’s specific stages of sleep have a beneficial dogs, who associated the ringing of a bell effect on memory, with promising with food and would salivate in response.6 1 conclusions being drawn. The relevance of this research is its potential to be translated Memory consists of three interdependent into clinical practice. This review aims to processes: encoding, consolidation and discuss the current evidence regarding the retrieval. In the process of encoding, new effect of sleep on different memory sub- information inputs into neural circuits. This classifications and the possible practical information is unstable and must be implications, to elucidate whether it strengthened and transferred to long-term strengthens or refutes our hypothesis that storage in the process of consolidation, sleep plays a pivotal role in memory which this project will focus on. Finally, this consolidation. The scope of this review is to information is retrieved from the areas of provide a condensed yet relevant summation storage.2 of this rapidly developing field of research for the curious medical student or doctor. Sleep can be divided into two phases: non- rapid eye movement (NREM) and rapid eye Memory is defined as the ability to encode, movement (REM). NREM consists of four consolidate and retrieve information that has stages: stages 1, 2, 3 and 4. Stages 3 and 4 are been learnt.2 It is categorized based on referred to collectively as slow wave sleep whether the retrieval of information is (SWS) as both exhibit a slow wave EEG conscious or subconscious, the former pattern.7 termed declarative (or explicit) memory and the latter termed non-declarative (or On average, one night of sleep involves four implicit) memory.1,3 or five sleep cycles. Sleep cycles are the progression through various stages of Declarative memory is further divided into NREM sleep and REM sleep, each lasting episodic memory, described as being the close to 90 minutes.8 The stages of NREM ‘diary’, and semantic memory, described as do not necessarily occur in order and vary being the ‘dictionary’4; episodic memory throughout sleep as illustrated in Figure 1. comprises autobiographical events whereas semantic memory comprises facts.5 Methods Non-declarative memory can be split into Research regarding the effect of sleep on the procedural memory, conditioning, non- different memory sub-classifications was associative memory and priming. This identified by searching for both primary review will focus on procedural memory and research and systematic reviews up until conditioning as little research exists March 17, 2013. Volume 21, Issue 1 23 Res Medica A sleep to remember: the effects of sleep on memory A literature search was carried out using the minutes after encoding, compared to Medline database and Google Scholar with subjects who were not allowed a nap.10 This the following keywords: result is in line with many studies that suggest a possible role for sleep in episodic “Memory” AND “Sleep” memory consolidation. “Memory” AND “Sleep” limited to meta- Although sleep as a whole has been shown analysis or review or systematic review to aid episodic memory consolidation,11 it is “Episodic memory” AND “Sleep” thought that certain sleep stages are more “Semantic memory” AND “Sleep” beneficial than others. However, exclusively “Declarative memory AND “Sleep” depriving specific sleep stages is impractical “Procedural memory” AND “Sleep” in humans, as this could either lead to REVIEW “Conditioning” AND “Sleep” disruptions in other sleep stages or affect “Non-declarative memory” AND “Sleep” potentially important sleep stage 12 transitions. Plihal and colleagues MeSH terms were used when available. discovered that the first half of one night’s Search terms were kept as broad as possible sleep involves almost five times more SWS to ensure that important literature was not than REM sleep, while the second half is missed. No language restrictions were put in associated with twice as much REM sleep as place. Articles were selected based on the SWS (Figure 2).13 This concept allows relevance to our subject topic and aims. An investigation of the effects of certain sleep article was deemed relevant according to the stages by separating test groups into ‘early title abstract and subsequent critical sleep’ and ‘late sleep’ groups, depending on appraisal of the text. whether the post-learning sleep occurs in early stages of the night or the late stages, Episodic Memory respectively. Therefore, by either preventing early or late sleep, the effects of SWS or Episodic memory involves the recall of REM sleep individually can theoretically be 13 events occurring in a particular place and determined. time.9 The effects of sleep on episodic memory consolidation can be explored Using this methodology, and a sample size through sleep deprivation and are objectively of 20, Plihal and Born documented quantified by using various episodic memory improved performance in an episodic tests. memory task in subjects assigned to the ‘early sleep’ group compared to those How does sleep affect episodic assigned to the ‘late sleep’ group, concluding memory? that SWS was more important in the consolidation of episodic memories than It is generally accepted that sleep enhances REM sleep.13 These results, and those of episodic memory consolidation. For other studies, support the dual-process example, van der Helm demonstrated hypothesis, which suggests SWS is necessary significant memory improvement in subjects for declarative memory whereas REM sleep who were allowed a two-hour nap 45 is essential for procedural memory.14-1 Volume 21, Issue 1 24 Res Medica Sarode DP et al. http://journals.ed.ac.uk/resmedica REVIEW Figure 1: Categorization of sleep stages. (Stickgold et al, 2005)1 Figure 2: Early sleep contains a high proportion of SWS compared to late sleep which contains a high proportion of REM sleep (Diekelmann S, et al. 2010)37 37 Volume 21, Issue 1 25 Res Medica A sleep to remember: the effects of sleep on memory Conversely, there have also been studies a part both in memory and sleep systems. demonstrating a correlation between REM During sleep, acetylcholine is at low levels sleep and episodic memory performance. during SWS and at high levels during REM Rauchs showed that ‘late sleep’, associated sleep. with mainly REM sleep, correlated with improved episodic memory performance It was shown that increasing synaptic compared to subjects in the ‘early sleep’ and acetylcholine concentration during SWS led ‘no sleep’ groups.17 Despite these results to poorer episodic memory consolidation.20 being ‘preliminary’ with further research Therefore, the effects of fluctuating being required in humans, these findings are chemicals during sleep, such as acetylcholine REVIEW incongruent with the dual-process and cortisol, could be potential confounding hypothesis, and suggest that REM sleep may factors to the above evidence.21 indeed play a role in episodic memory consolidation. Semantic Memory A possible explanation for the above conflict in evidence (solely SWS or solely REM sleep Semantic memory encompasses stored ideas is essential) is the sequential hypothesis. This and concepts without the context of where hypothesis, suggesting alternation of sleep and when the information was formed.22 stages in cycles facilitates memory Specifically testing the individual aspects of 18 consolidation, was investigated and it was declarative memory can prove challenging proposed that SWS promotes integration of due to the episodic factors involved in new memories into pre-existing memory development of semantic memory. stores while REM sleep acts to stabilize these memories. However, evidence is How does sleep affect semantic lacking and further research is required for a memory? greater understanding of the mechanisms 19 involved. Generally, studies use sleep deprivation to test the effect on semantic memory of either What flaws and limitations are present sleep as a whole or of certain sleep stages.