Over the past two decades, research has accumulated compelling evidence that sleep supports the formation of long-term memory. The standard two-stage memory model that has been originally elaborated for declarative memory assumes that new memories are transiently encoded into a temporary store (represented by the hippocampus in the declarative memory system) before they are gradually transferred into a long-term store (mainly represented by the neocortex), or are forgotten. Based on this model, we propose that sleep, as an offline mode of brain processing, serves the ‘active system consolidation’ of memory, i.e. the process in which newly encoded memory representations become redistributed to other neuron networks serving as long-term store. System consolidation takes place during slow-wave sleep (SWS) rather than rapid eye movement (REM) sleep. The concept of active system consolidation during sleep implicates that (a) memories are reactivated during sleep to be consolidated, (b) the consolidation process during sleep is selective inasmuch as it does not enhance every memory, and (c) memories, when transferred to the long-term store undergo qualitative changes. Experimental evidence for these three central implications is provided: It has been shown that reactivation of memories during SWS plays a causal role for consolidation, that sleep and specifically SWS consolidates preferentially memories with relevance for future plans, and that sleep produces qualitative changes in memory representations such that the extraction of explicit and conscious knowledge from implicitly learned materials is facilitated.

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The brain encodes huge amounts of information, but only a small fraction is stored for a longer time. There is now compelling evidence that the long-term storage of memories preferentially occurs during sleep. However, the factors mediating the selectivity of sleepassociated memory consolidation are poorly understood. Here, we show that the mere expectancy that a memory will be used in a future test determines whether or not sleep significantly benefits consolidation of this memory. Human subjects learned declarative memories (word paired associates) before retention periods of sleep or wakefulness. Postlearning sleep compared with wakefulness produced a strong improvement at delayed retrieval only if the subjects had been informed about the retrieval test after the learning period. If they had not been informed, retrieval after retention sleep did not differ from that after the wake retention interval. Retention during the wake intervals was not affected by retrieval expectancy. Retrieval expectancy also enhanced sleep-associated consolidation of visuospatial (two-dimensional object location task) and procedural motor memories (finger sequence tapping). Subjects expecting the retrieval displayed a robust increase in slow oscillation activity and sleep spindle count during postlearning slow-wave sleep (SWS). Sleepassociated consolidation of declarative memory was strongly correlated to slow oscillation activity and spindle count, but only if the subjects expected the retrieval test. In conclusion, our work shows that sleep preferentially benefits consolidation of memories that are relevant for future behavior, presumably through a SWS-dependent reprocessing of these memories.

Sleep supports the consolidation of memory in adults. Childhood is a period hallmarked by huge demands of brain plasticity as well as great amounts of efficient sleep. Whether sleep supports memory consolidation in children as in adults is unclear. We compared effects of nocturnal sleep (versus daytime wakefulness) on consolidation of declarative (word-pair associates, two-dimensional [2D] object location), and procedural memories (finger sequence tapping) in 15 children (6-8 yr) and 15 adults. Beneficial effects of sleep on retention of declarative memories were comparable in children and adults. However, opposite to adults, children showed smaller improvement in finger-tapping skill across retention sleep than wakefulness, indicating that sleep-dependent procedural memory consolidation depends on developmental stage.Compelling evidence has been accumulated that sleep supports the consolidation of newly acquired memories in adults (Maquet 2001;Stickgold 2005;Born et al. 2006). Declarative memories benefit particularly from slow wave sleep (SWS), whereas procedural memories benefit particularly from REM sleep (Plihal and Born 1997;Peigneux et al. 2004;Marshall and Born 2007), aside from distinct contributions of non-REM sleep stage 2 to memory consolidation (Gais et al. 2002;Fogel and Smith 2006;Peters et al. 2007). Childhood, compared with adulthood, is characterized not only by distinctly greater amounts of sleep and SWS (Anders et al. 1995;Ohayon et al. 2004) but also by a tremendous extent of brain and behavioral plasticity, determining the child's capability to rapidly acquire huge amounts of facts and to effectively shape skills in response to environmental challenges (Li et al. 2006;Brehmer et al. 2007). However, the role developmental sleep plays for consolidating memory has only been scarcely examined. Restriction of sleep in schoolchildren was shown to be associated to impairments in different cognitive functions (Carskadon et al. 1981;Randazzo et al. 1998;Steenari et al. 2003). Animal studies provided considerable evidence that developmental sleep, like sleep in adults, is crucially involved in brain plasticity (for review, see Dang-Vu et al. 2006). However, in a recent human study (Fischer et al. 2007), children aged 7-11 yr, in contrast to adults, showed impaired rather than improved implicit sequence knowledge in a procedural serial reaction-time task when training was followed by periods of sleep, pointing toward differential dynamics of sleep-dependent consolidation of procedural memories during development.In the present study, we dissociated effects of post-learning sleep on procedural and declarative types of memories in 15 healthy children (age 6-8 yr, mean ‫ע‬ SEM: 7.5 ‫ע‬ 0.16 yr; 9 females, 6 males) and 15 healthy adults (26.5 ‫ע‬ 1.3 yr; 13 females, 2 males). The study was approved by the local ethics committee, and informed consent was obtained by participants and the children's parents. All subjects had normal sleep and were adapted to standard polysomnographic recordings (obtained by a...

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Sleep consolidates memory and promotes generalization in adults, but it is still unknown to what extent the rapidly growing infant memory benefits from sleep. Here we show that during sleep the infant brain reorganizes recent memories and creates semantic knowledge from individual episodic experiences. Infants aged between 9 and 16 months were given the opportunity to encode both objects as specific word meanings and categories as general word meanings. Event-related potentials indicate that, initially, infants acquire only the specific but not the general word meanings. About 1.5 h later, infants who napped during the retention period, but not infants who stayed awake, remember the specific word meanings and, moreover, successfully generalize words to novel category exemplars. Independently of age, the semantic generalization effect is correlated with sleep spindle activity during the nap, suggesting that sleep spindles are involved in infant sleep-dependent brain plasticity.

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When sleep followed implicit training on a motor sequence, children showed greater gains in explicit sequence knowledge after sleep than adults. This greater explicit knowledge in children was linked to their higher sleep slow-wave activity and to stronger hippocampal activation at explicit knowledge retrieval. Our data indicate the superiority of children in extracting invariant features from complex environments, possibly as a result of enhanced reprocessing of hippocampal memory representations during slow-wave sleep.