The Effects of Sleep on Prospective Memory: a Systematic Review and Meta-Analysis
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Sleep Medicine Reviews 47 (2019) 18e27 Contents lists available at ScienceDirect Sleep Medicine Reviews journal homepage: www.elsevier.com/locate/smrv CLINICAL REVIEW The effects of sleep on prospective memory: A systematic review and meta-analysis * Ruth L.F. Leong a, Grand H.-L. Cheng b, Michael W.L. Chee a, June C. Lo a, a Centre for Cognitive Neuroscience, Neuroscience and Behavioral Disorders Program, Duke-NUS Medical School, Singapore b Centre for Ageing Research and Education, Duke-NUS Medical School, Singapore article info summary Article history: Prospective memory (PM) enables us to execute previously conceived intentions at a later time and is Received 23 January 2019 used when remembering to call a friend or submitting a proposal on time. Evidence that sleep benefits Received in revised form PM is presently mixed. Further, when a benefit is observed, it is unclear if this is achieved through 23 May 2019 improvements in strategic monitoring (maintaining an intention in mind and searching for cues) or Accepted 27 May 2019 spontaneous retrieval (an automatic process occurring without preparatory attention). We conducted a Available online 31 May 2019 meta-analysis of 24 independent samples (N ¼ 165,432) to quantify the effect of sleep on PM and gain clarity regarding the retrieval process benefitted by sleep. Cohen's d with 95% confidence intervals (CI95) Keywords: fi fi Sleep were derived using random-effects models. The bene t of sleep on PM was statistically signi cant and in ¼ ¼ e fl Prospective memory the small to medium range (d 0.41, CI95 0.25 0.56). Moreover, sleep did not appear to in uence Memory consolidation monitoring (d ¼0.11, CI95 ¼0.40e0.17). In contrast, the benefits of sleep are significantly greater Spontaneous retrieval when the likelihood of spontaneous retrieval is high (d ¼ 0.94, CI95 ¼ 0.44e1.44) versus low (d ¼ 0.45, Monitoring CI95 ¼0.02e0.93), suggesting that sleep may leverage on spontaneous retrieval processes to improve PM. These findings inform theoretical models of sleep and PM that could sharpen strategies to improve memory function in vulnerable populations. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Introduction Recently, a number of studies have shed light on the importance of sleep in PM: sleeping after encoding an intention improved PM Prospective memory (PM) refers to the ability to self-initiate [5e8], whereas short sleep [9e14] and sleep deprivation [15e17] previously conceived intentions and this enables us to execute were associated with impaired PM. However, other studies re- future goals [1]. These intentions can be executed at a specific time ported no benefits of sleep over wake on PM [18e20] and no sig- (e.g., attending a meeting at 9 am) e time-based PM e or in nificant differences in PM between individuals with disturbed sleep response to the occurrence of an event (e.g., relaying an important and healthy controls [21,22]. Hence, the first aim of this study was message to a colleague the moment he arrives at the meeting) e to conduct a meta-analysis on previous work to collectively assess event-based PM. PM tasks are ubiquitous in everyday situations, the effect of sleep on PM. failures of which comprise 50e80% of all memory oversights Another outstanding question regarding sleep and PM is the experienced everyday [2e4]. As PM failures can have catastrophic extent to which the latter is facilitated through strategic monitoring consequences, e.g., forgetting an insulin injection, it is important to and/or spontaneous retrieval processes. In the present work, we study antecedents of PM. adopt the Multiprocess framework [23,24] to examine retrieval processes. According to this framework, monitoring involves the allocation of attentional resources to keep the intention active in mind (e.g., holding in mind the intention to pass the message to Abbreviations: CI, confidence interval; PRISMA, preferred reporting items for your colleague) as well as to look out for the PM cue (e.g., attending systematic reviews and meta-analysis; PFC, prefrontal cortex; PM, prospective to the door in anticipation of your colleague's arrival). In contrast, memory; REM, rapid eye movement; SD, sleep deprivation; SWS, slow wave sleep; spontaneous retrieval is a reflexive associative process whereby TST, total sleep time. ‘ ’ * Corresponding author. Centre for Cognitive Neuroscience, Duke-NUS Medical intentions simply pop into mind . For example, registering that a School, 8 College Road, Level 2, 169857, Singapore. colleague has come through the door triggers automatic retrieval of E-mail address: [email protected] (J.C. Lo). https://doi.org/10.1016/j.smrv.2019.05.006 1087-0792/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). R.L.F. Leong et al. / Sleep Medicine Reviews 47 (2019) 18e27 19 the intention to pass on a message [23,24]. Both retrieval processes “Sleep Medicine”, “Journal of Clinical Sleep Medicine”). We also may be engaged within the same task [25], and task conditions can examined reference lists within relevant published articles to influence the likelihood of spontaneous retrieval or monitoring locate other studies. processes. For unpublished works, we searched for theses and disserta- Notably, a role for sleep in facilitating spontaneous retrieval has tions on the databases OATD.org (open access theses and disser- been proposed [5,7,8]. This is of particular relevance to persons who tations) and ProQuest using the same keywords and combinations. have limited attentional resources for holding an intention in mind We did not find any unpublished studies examining the association while monitoring for the PM cue, for example, someone who is between sleep and prospective memory in healthy individuals and busily engaged in multiple tasks throughout the day, or an older individuals with sleep disorders. adult with diminished attentional capacity. However, few studies have explored this by successfully contrasting sleep's effects on the Inclusion and exclusion criteria two retrieval processes, a limitation which is compounded by the complexity of isolating spontaneous retrieval from monitoring [26]. We included studies into our meta-analyses that fulfilled the It is difficult to definitively conclude which retrieval process is following criteria: 1) original article; 2) sleep as a) an independent responsible for successful prospective remembering without cor- variable examined as a primary manipulation, b) a group contrast responding physiological indices to confirm the underlying involving a sleep disorder group and a healthy comparison group, neurological process e unfortunately, all present studies on sleep or c) a continuous measure assessed with either objective or self- and PM lack this feature. reported measures; 3) objective or self-reported assessment of Nonetheless, here, as a first attempt to examine which PM PM; 4) report of sufficient descriptive or inferential statistics that retrieval process sleep would preferentially benefit, we utilized could be converted to Cohen's d. We contacted authors to request available data from the literature and adopted the following for the relevant statistical information if it was not reported in the reasoning: if an observed sleep improvement was not accompanied published article. We excluded studies that examined sleep in in- by increased monitoring, it may have been facilitated by sponta- dividuals with clinical conditions (e.g., multiple sclerosis) or neous retrieval. However, as spontaneous retrieval cannot be cognitive impairments (e.g., mild cognitive impairment) as these measured directly, we asked instead whether sleep's benefitwas might have contributed to deficits in PM. greater when the likelihood of spontaneous retrieval was higher, For studies including multiple measures of sleep or PM, our which would suggest that sleep may be tapping on spontaneous selection of findings for final inclusion into the meta-analysis was retrieval processes to produce an enhancement of PM. Hence, we guided by the principle of having a sufficiently broad representa- first sought out studies that showed a sleep benefit. A summary tion of the multifaceted concepts of ‘sleep’ and ‘PM’. This was to effect size was then computed for sleep's effect on monitoring. ensure that our meta-analysis would not be biased by a particular Next, for studies in which significantly better PM performance in operationalization of either term. the sleep group was not accompanied by significantly increased Ohayon et al. [30] reported the effects of both short (5 h) and ongoing costs compared to the wake group, we examined whether long habitual sleep (>8.5 h) on PM in older adults using the same the effect size of sleep on PM would be greater when the likelihood reference group (7e8.5 h). As meta-analyses require independence of spontaneous retrieval was high versus low. This approach of effect sizes, we included the effect size for the effect of long sleep enabled us to estimate the extent to which sleep's effects are instead of short sleep for two reasons: 1) the sample size for that boosted when spontaneous retrieval processes are more easily group was larger (n ¼ 138 versus n ¼ 113), and 2) in healthy older accessed. adults, habitual long sleep is less frequently examined in the liter- Lastly, we explored whether the effect of sleep on PM varied ature compared to habitual short sleep. Fine et al. [31] examined depending on study factors, such as, sample age and PM type. the association between several sleep parameters and PM, such as Improvement of memory may vary with age [27,28] and we total sleep time (TST), sleep efficiency, wake after sleep onset, extended this evaluation to PM. PM tasks may be time-based or awakening length, and number of awakenings.